I alluded to this in the Pick a Size article, but let's make sure that you're thinking clearly if you're trying to pick between the various cameras. Here are the factors I would consider before picking a mirrorless system:
- What do you already have? If you're already an Canon EOS, Nikon F-mount, Olympus 4/3, or Sony Alpha user, there are some advantages to staying within the brand. Some of your lenses may be perfectly usable with the right lens adapter (though they may not have the same angle of view on the mirrorless camera). Nikon is probably the most anal in keeping retro compatibility, but Sony is a close second. Canon with the EOS M has chosen to mimic the Sony/Nikon approach. This item isn't usually a deciding factor (you'll typically still need some new lenses, especially wide angle), but it sometimes is a tipping factor towards one system or another.
- What will you grow into? These are system cameras. That means that there are lenses, flash units, and other accessories that augment the basic camera itself. Heck, in most lineups there are multiple cameras to choose from: you can start with a low-end one and get a more sophisticated model if you find you need more. So think a bit about what you're doing photographically now, and what you might want to do in the future. Make sure that the system you pick supports (or will support—some of these systems have Road Maps that indicate future augmentations) what you think you'll need. Buying into a system that doesn't have what you need now and has provided no indication that they'll address that need in the future is a gamble.
- What do you shoot? Are you a JPEG shooter (ready-to-use images) or are you a raw shooter (images require post processing)? JPEG shooters should pay some attention to how good the in-camera processing really is. For example, pretty much from the beginning the quality and impact of the Oympus JPEGs has been better than the Panasonic JPEGs, even from the same sensor! If all else were equal and I was a JPEG shooter, I'd tend to pick an Olympus model over a Panasonic. (Please note the dependent clause "if all else were equal”; also note that Panasonic has slowly but surely closed this gap, though automatic white balance still trips them up). Nikon has typically had very neutral but very clean JPEG output. Sony has tended to have heavy-handed noise reduction at high ISO values. There's no "right" in-camera processing, only "different." Check out one of the sites that perform extensive side-by-side testing (e.g. DPReview), but also note that most of these sites test at camera default settings. In many cases you can get better results than tests indicate by tweaking settings. But that takes time and energy to perform. Make sure you have that time.
Raw shooters worry about what is the best they can get out of the sensor data. There's no clear answer to that, and the answer changes over time. I can post process images I took 10 years ago better than I could 10 years ago, both because I've learned more and the tools I use have gotten better. While raw shooters seem to think they can pick a camera based upon pixel examination or extreme engineering testing (e.g. DxoMark type data), in practice I've found few people who really understand what it is that they're looking at. Let me put it this way: I've shot with all of these cameras, and I've been able to get very good results with all of them while shooting raw. But that was generally a process of trial-and-error discovery. Personally, I think that we're in an era where virtually all of the sensors can provide solid data streams in reasonable light. Yes, the smaller the sensor is the more it might struggle in low light, though even there things are changing. If you're a perfectionist, sure, get lost in the pixel details. But if you're a photographer, understand that no matter which system you pick, it will have limits, and those limits won't be all that far off from the other systems you could have picked.
- How low do you go? Low-light is an issue we have to talk about. Despite what I just wrote in the last point, the Sony A7 raws have a lot of data in them and the data is well-mannered even into some of the higher ISO values. The m4/3 cameras tend to not perform as well in low light (I use an upper boundary of ISO 800 on older models, 1600 for the latest models). The Pentax Q and Q10 are definitely not low light cameras.
If you're a low light shooter, the generalization is that "bigger sensors will be better for you." But be careful here. Using an f/5.6 lens with a larger sensor (or a sensor that performs well in low light) is not what a low light shooter should be doing. They actually may be better off with a smaller sensor and an f/1.4 lens. Right now, this kind of disaparity exists, in particular between the m4/3 and APS models: we've got lots of low-light lenses in the m4/3 lineup, few in the APS lineups. But we've got better low-light handling in the APS sensors but not in the m4/3 ones. Personally, I'd rather have the lens first, and a better sensor later rather than a better sensor and wait for the lens I need. It's a tough choice but it's one you have to consider if you're a low-light shooter.
- How will you carry it? Traditionally, SLR and DSLR cameras were carried via neck straps, and complete systems were usually carried in big backpacks. The smaller, lighter mirrorless cameras open up some new possibilities. I once went on a 13-mile hike where I simply carried my m4/3 camera in my hand. It was that small and light, and that also meant it was always popping up to my face to take pictures. In some ways, it was a transformative experience. I found myself examining a lot more potential shooting situations. True, the camera was fully exposed to my possibly dropping it or bumping it into something, but as a professional photographer I value accessibility over longevity.
Some of these systems can be incredibly small. Small enough that you fit an entire system into a small bag, or even into your jacket or vest pockets. Other mirrorless systems tend to have bigger lenses that preclude that. So pay attention to all the ways you might want to carry your camera.
All that said, I've been saying for quite some time that if you can't get good-looking prints at the maximum size a desktop inkjet printer achieves (13x19"), then it isn't the camera that's the issue. Other than really low light work, I'd say that applies to all the mirrorless systems from the Nikon 1 size on up.
What I usually say to someone is this: if you pick these cameras up in a store and just give them a quick test drive you're going to have a visceral and natural response to one of the systems over the others. Pay close attention to that response, as it's telling you something.
In serious photography we worry about "the moment." The way a camera handles and the location of the controls can get in the way of capturing the moment. If you're fiddling with buttons or dials or looking for the right control to change, the picture is long gone. Some people respond to certain types of user interface controls better than others. If you're young and used to touchscreen devices, a touchscreen interface may feel perfectly natural to you. If you're an older shooter who used Nikon film SLRs and gravitated to their DSLRs, you're used to a button-and-dial interface (surprisingly, the Nikon 1 doesn't share that interace, but some of the cameras from other makers do).
Above all, choose a camera you either feel you're already comfortable with or are sure you can get comfortable with, and who's system will let you grow into the other lenses and accessories you might desire.