Each system has its own strengths and weaknesses:
- Strengths: small size, compatibility with EOS DSLRs
- Weaknesses: lens choices, low-light performance, focus speed
- Strengths: lenses, body choice, controls, autofocus performance
- Weaknesses: video
- Strengths: video, sensor, autofocus performance, frame rate, F-mount compatibility (with adapter), novice user orientation
- Weaknesses: lens choices, low-light performance, overall size versus sensor performance, high-end user control (except V3), 2.7x crop sensor size, price
- Strengths: video (Panasonic), autofocus performance, lens choice, range of available models, sensor performance (lower ISO values)
- Weaknesses: low-light performance still weaker than larger sensors, price
- Strengths: photographer-centric body design, very small system
- Weaknesses: lens choices, sensor performance, low-light performance, lack of pixel count
- Strengths: photographer-centric designs, APS sensor size
- Weaknesses: lens choice
- Strengths: APS sensor size, sensor quality, low-light performance, Alpha mount compatibility (with adapter), small size for sensor size
- Weaknesses: size of lenses, lens choice (improving slowly), tendency towards overheating
- Strengths: full frame sensor size, sensor quality, low-light performance (esp. A7s), Alpha mount compatibility (with adapter), small size/weight/price for full frame
- Weaknesses: size of lenses, lens choice (improving slowly), raw sensor data crippled slightly, shutter slap on original A7r
As with DSLRs, there are advantages to staying within an ecosystem (Nikon, Olympus, Sony, etc.). In particular, the Canon EOS, Nikon 1, and Sony E/FE-mount models have good compatibility with their DSLR lenses via adapter (4/3 adapted to m4/3 tends to have poorer autofocus performance, except on the E-M1). 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 was 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 D810 and 14-24mm lens isn't really a burden. However, when I go seriously into the back-country, having an E-M10 with a 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.