New Mirrorless - What Should You Buy

Sony A7mII, lens unrecorded

So, which of the new FF mirrorless systems should you buy.

Sony Shooter - Investment in E, FE or A lenses - Just stick with Sony unless you really need a line of f1.8 primes. You're already invested in the most mature system and neither of the other options provides enough advantages to offset your current investment.

Sony Shooter - Investment in adapted lenses - Take a close look at the Nikon Z's if you aren't on an A7RIII or A9, as their non-CPU lens data interface solves a major workflow issue for adapters and the Z6 EVF is decidedly better than the A7III (the A7RIII and Z7 are comparable for EVF quality). You'll want to wait for adapters to show up unless your lenses are primarily F mount.

Nikon Shooter - Modern AF lenses - Look at the Nikons, they're a no brainer unless you need speed for wildlife/event photography, or dual card slots, at which point wait for the Z8/Z9 which will inevitably follow

Nikon Shooter - Screwdriver AF lenses - Sucks to be you, if you need to move, it's time to invest in newer glass at which point you may want to look at the more complete Sony system

Nikon Shooter - MF lenses - Nikon's the best option here, but Sony is very viable as well, and faster plus equipped with dual card slots and better battery life. If you're a Zeiss fanatic, the Batis & Loxia lenses argue for Sony.

Canon Shooter - EF or EF-S lenses - Buy a Sony + MC-11 or Metabones adapter for the best experience. Only reason to get the R is weather sealing.

Canon Shooter - EOS M shooter - Buy a Sony or Fuji if you don't adapt EF lenses. Canon has abandoned you in terms of any ability to keep your investment as you move up in cameras.

Note I don't see any category where there's a real argument for the EOS R. It's not a bad camera, but there's no use case other than Canon Diehard where it's the best option.

For me, I'm in the Nikon Shooter - MF lenses category for the most part. And that means that I'm in the one area where there is no really set differentiation there.