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Which camera should I buy?
Each system has its own strengths and weaknesses:
- Strengths: video, sensor, autofocus performance, F-mount compatibility (with adapter), novice user orientation
- Weaknesses: lens choice, low-light performance, overall size versus sensor performance, high-end user control, range of available models, 2.7x crop sensor size
- Strengths: video (Panasonic), autofocus performance, lens choice, range of available models
- Weaknesses: sensor performance, low-light performance
- Strengths: photographer-centric body design, modularity, Leica M-mount compatibility, APS sensor size
- Weaknesses: cost of lensors (sensor+lens modules), lensor choices, lack of dealers, video capability
- Strengths: photographer-centric designs, APS sensor size
- Weaknesses: sensor performance, lens choice (though improving)
- Strengths: APS sensor size, sensor quality, low-light performance, Alpha mount compatibility (with adapter), user control (NEX 7)
- Weaknesses: size of lenses, user control (NEX 3, 5), lens choice (improving slowly), tendency towards overheating
As with DSLRs, there are advantages to staying within an ecosystem (Nikon, Olympus, Sony, etc.). In particular, the Nikon 1 and Sony NEX models have very good compatibility with their DSLR lenses via adapter (4/3 adapted to m4/3 tends to have poorer autofocus performance). So if you own a DSLR from a brand, the first place you should look is at the mirrorless camera from the same brand. Only if there's something that doesn't suffice in that mirrorless camera (the lack of user control on the Nikon 1 models, for example) should you look at out-of-brand solutions, in my opinion.
That said, I've been basically two-brand since the beginning of mirrorless: I have full Nikon F-mount systems (both FX and DX) and m4/3 mirrorless systems. One reason for this was that m4/3 was the first truly smaller system for which I was able to build a near duplicate lens set. For example: my Nikon 14-24mm on FX is matched by Panasonic 7-14mm on m4/3. Yes, I don't have aperture equivalence, but it does allow me to stay shooting with the same approach with both m4/3 and FX in my landscape work. If, for example, I'm mostly shooting in front country, carrying a heavy, bulky D3x and 14-24mm lens aren't really a burden. However, when I go seriously into the back-country, having an E-P3 with the 7-14mm allows me to capture many of the same images while carrying a far smaller and lighter kit. When you're trying to hike 16 miles on tough trails in a day, the size/weight difference is indeed something that is meaningful.
Bottom line, there's no easy answer to this question. You have to evaluate your needs and wants carefully against what's available and pick the system that works best for you.