Despite the viability of the Nikon D7100 and Canon 70D, I think that anyone serious enough to buy a camera body between US$1000 and US$2000, will be wanting the most functionality and the least weight to take photography seriously. Sure, you can buy older, cheaper lenses for the D7100 and 70D, but the newer, for-digital lenses cost quite a lot, and they're big.
The E-M1 and X-T1 are similar in many ways. They're loosely based on the company's SLRs of the 1970s. They have a good group of lenses made for the digital world. They have awesome electronic viewfinders and a sliding rear panel.
FujiFilm has that great X-Trans II sensor with its unconventional, non-Bayer pattern color filter, so even though both bodies have roughly 16 MP available in photos, FujiFilm's sensor should give better results. Of course, a lot of this depends on the output and processing. So far, raw processing has not been great, as development companies struggle to learn how to deal with the new patterns. (Phase One Capture One 7 Pro seems to be better than most and Adobe seems to be worse than most.) Given that Olympus is using a typical color filter on its sensor, it's much easier to get great results from raw development applications processing the files. As software developers acclimate themselves to the color attributes, the FujiFilm raw files should prove victorious.
Tests I've seen using large prints have shown that people can tell little difference between different sizes of sensors, even though advocates will tell you that the larger sensor is definitely much, much better. The truth is--it's difficult to find any current camera from micro Four-Thirds on up that will produce bad photos given similar circumstances.
Operationally, the X-T1 has the advantage of physical controls. There are many on the top plate. However, the E-M1 has the advantage of the Super Control Panel. It's very easy to make quick adjustments using the rear display. There is no room for a display on the top plate and it's just as well, seeing as how the display on the Nikon Df is not quite as useful as we could hope. Given how big the Df is over the X-T1 or E-M1, you'd think that there could have been a better compromise made by Nikon.
Having the exposure compensation on the top plate, easy to find, is a win, and as I had to search and search for the control on the E-M1, the X-T1 wins easily. I don't think having the ISO sensitivity control on the top plate helps a bit but FujiFilm has placed it there, as the film speed dial was there on the Fujica SLRs. It would be more helpful if the company would abolish their ISO sensitivity limitations for raw files. I'm not sure how useful any file will be at 51,200, but I would want to process such a file with a raw development application to smush (or schmush, if you prefer--that is a highly technical term for reduction) the noise (and not the image detail) in pleasing ways. Of course, back in the 1970s, we were generally restricted to ASA 400/DIN 27 film and 1/1000th of a second fastest shutter speed and a maximum aperture of f/1.2.
On available lenses, the E-M1 has the advantage with two major brands bringing lenses to market over a few years. FujiFilm is coming along with quite a few good focal lengths, while Olympus is making some interesting choices (the death of their Four-Thirds bodies gives way to HG-like lenses for micro Four-Thirds) nowadays and Panasonic is trying to use Canon's lenses as a guide. It's a bit difficult to tell the better lenses from Olympus, except for price and most recently, the "PRO" label. Panasonic's X label isn't exactly helpful, but that signals their best lenses, and FujiFilm is using XF to mark theirs. Considering third party lenses, FujiFilm has the advantage with electronically-enabled lenses, while Olympus has the advantage with lenses that have no more than a micro Four-Thirds mount. If Schneider and Tamron would actually produce the micro Four-Thirds lenses announced, there would be some interesting alternatives. Schneider, 2013 has come and gone and your 14mm f/2.0 lens doesn't even seem to be in the hands of testers.
Of course, handling will be the big deal for any camera body. As I've said too many times, SLRs from the 1970s were slippery. Mount a heavy lens and you'd better pay attention more than with a current body. The E-M1 and X-T1 have similar grips, and the E-M1 grip was sufficient for light, native lenses. Putting Four-Thirds lenses on the body made the grip uncomfortable for me. This isn't a problem for the X-T1, unless of course, you buy a third party adapter but then, that wasn't planned by the company, whereas the Four-Thirds lenses were meant to work with the E-M1, even though it still seems like an afterthought. You can buy a battery grip for each to help with balance and shooting time.
I found the electronic viewfinder of the E-M1 to be so good that I could give up an optical viewfinder. The viewfinder on the X-T1 is supposedly better, with less lag and a better refresh rate in lower light. The information displayed on the X-T1 in portrait orientation is usefully re-arranged.
If I were starting fresh, I'm not 100% sure what I'd pick but I suspect that it would be the X-T1.
Update 2014.02.17: I've been thinking how the Panasonic GH4 interrupts these thoughts. However, if still photography is my goal, these two will likely still be better choices. I viewed a video today by Guilio Sciorio, a Panasonic Lumix Luminary, and he commented that the GH4 is a much better shooter than the GH3. It's good that you can pull an 8 MP image out of a video file. I've also see Panasonic marketing materials saying that the image quality is better than many other bodies, but it didn't specify stills or video. The video quality is a given, but stills quality? I'm not so sure.
Update 2014.02.22: What Digital Camera gave the X-T1 a score of 93% and the E-M1, 92%. This is my go-to magazine, as I don't see advertising or personal preference influencing the outcome.
Some of the same things bother me, such as the rear display being out in the open, and only sliding or flipping, plus the battery is too small, though the E-M1 has fairly good battery life but the grip is uncomfortable (for me, especially with Four-Thirds lenses) because they're using a small battery and somehow they couldn't see to make it more like the E-5 or GH3. I thought both of these bodies needed two SD Card slots also. At the camera store the other day, they'd mentioned that the X-T1 bodies are trickling into their stores, and are already sold by the time they arrive. Obviously, the E-M1 has been available longer but it's being bought at a good pace, also, despite the size and price complaints from various micro Four-Thirds users.
Update 2014.03.01: It's March already? I spent a little time with an X-T1 yesterday. I didn't photograph anything, so I don't have photos to share. I wanted to compare the size and get a feel for how things worked in the confines of the store. It felt small, as the E-M1 feels small to me. The grip was smaller but better shaped. If they'd put a bigger battery in the grip, two of my doubts would have been handled at once. Given that I wouldn't be putting huge lenses on it, I don't think I'd have as much of a comfort issue as I would with the E-M1 and Four-Thirds lenses.
This blog entry is all about starting fresh, and I think either one is a good choice. The number of manual controls is over the top on the X-T1. I'm not sure if that is a good thing. I don't frequently change the ISO sensitivity setting, so I don't need a manual control just for that, just as it was in the film days, when they had it accessible but somewhat secondary to the shutter speed.
|The 1970s had simplicity on their side|
|Lift and Turn to set the film speed|
|Manual controls for practically everything possible|
|X-T1 and GH3|
The build was solid, similar to the E-M1. It felt better than the Canon 70D or the Nikon D7100--its main rivals, although the grip (and battery inside the grip) was the advantage for the dSLRs.. Once the weather-sealed lenses are available, it should be a very potent package.
Update 2014.03.20: I've seen a lot of good reviews about the X-T1. However, a lot of those seem to be people enamoured of the sheer number of physical controls on the top plate, not necessarily the actual performance, which seems to be very much like the X-E2. The X-E2 is very good, mind, but it's a bit sluggish (and the X-T1 is similar) compared to the E-M1 or the GH3 or the D7100 or the 70D. FujiFilm has come a long way toward making their system desirable. For casual users, I think it should be number 1 because it gives you great images with a minimum of fuss and the film simulation modes give people more of an idea of what to expect than Standard, Natural, or Vivid would suggest. The top shutter speed of 1/4000th of a second seems a bit limiting, especially with the 56mm f/1.2 lens. Neutral Density filters come to mind, though.
That said, the E-M1 is very quick. For sports, it's easy to choose the E-M1, even over the D7100 or 70D. That 1/8000th of a second shutter speed is helpful in many situations, as I have found with the Olympus E-5. The E-M1 is not a body for the casual user. The E-M5 is a better choice for casual users and it is less expensive.
Update 2014.11.06: I'm thinking about the X-T1 again. There is a Rokinon (Samyang/Bower...) 8mm f/2.8 fisheye lens that is available for FujiFilm X-Mount cameras.
The 135 Format equivalent would be 12mm--plus this is an f/2.8 lens, not f/3.5 and that is somewhat more useful. Add to that FujiFilm's sensor handling low light a bit better than Olympus', there are possibilities that I could get some skate park photos in lower light, with higher image quality. It's not guaranteed but it's possible.
The only thing that worries me is that the X-T1 might not be responsive enough. That was always my worry. The E-M1 is extremely responsive, as are the GH3 and GH4. FujiFilm seems to save money by not putting strong processors in the camera bodies. For many people, this isn't a problem. Casual photographers won't have a problem with it but sports photography doesn't wait for you to futz with your camera. When I tried the X-T1, I had wished for a running event outside the store, so I could give it a good workout. Unfortunately, the lenses I want/need for sports still aren't available.
Rokinon seems to have several lenses available, and even though they're manually-operated lenses, they should be good for FujiFilm users. There are more available for micro Four-Thirds though they're not all of the same lenses.
I'm still considering what's best for me.