Full Frame - As with a DSLR, a full frame sensor will give you much better performance in all respects as compared to a crop sensor. Especially High dynamic range and great performance at higher ISOs
4K - This is the resolution for video that is soon going to be the standard, if not already and many fields. Do remember that the file sizes of raw 4k videos as you shoot are huge and you require a bigger and faster memory card as well.
Viewfinder -Mirrorless cameras do not have optical viewfinders. If the viewfinder is absent, you will not be able to shoot with the camera closer to your eye as you do with DSLRs. Having a viewfinder - electronic or rangefinder type is certainly a plus point.
Tiltable screen - Its good to have a tiltable screen as that enables you to shoot from many possible angles otherwise not very convenient to shoot. Vertically rotating (upward-downward) screen is better than the one which rotates sideways.
Megapixel - More is always better.
Weight and compactness - You are buying a mirrorless camera because you do not want to move around with a heavy camera. Go for lighter weight camera otherwise the very objective of buying a mirrorless is defeated.
There are very limited options today (september 2017) in terms of MIrror less cameras which have full frame sensor along with 4K shooting capability.
Sony Alpha a9
Leica SL (Typ 601)
Between the two sony cameras Alpha a9 overall has better features if you are into sports, street, wildlife or any other genre which requires a fast and continuos shooting along with more number of focusing points. Higher shutter speed of 1/32000 is great for outdoors. If higher resolution is the priority then 42mega pixel will certainly give you far more detail than 24.2 mega pixel.
|Better in Alpha a9||Better in 7R II|
|20fps continuos shooting||5fps|
|1/32000 shutter speed||1/8000|
|693 phase detection autofocus points||399|
There are other mirror less cameras with 4K but they don't have a full frame sensor.
In my opinion, go for Sony a9 if you are on the go and need a fast camera with the ease of faster focusing. If you shoot in the studio and under controlled conditions having a better resolution might actually be your priority. Go for a7R II, which will in no way disappoint you outdoors in demanding situations.
I am currently shooting on Phase one - Mamiya - canon systems.
Both Nikon and Canon DSLR cameras are also good but when it comes to mirrorless cameras I think Sony is the best available option with both 4K and full frame capabilities. In days to come, if I add a mirrorless camera, it probably would be either sony a9 or Sony Alpha a7 R II
The prices may not drop but technology certainly will get more perfect with passage of time.