I've decided to skip the 5DIII upgrade and wait to see if the 5D4 will be a big enough leap forward to even consider upgrading. Quite happy with the Mk2 right now.
So I choose "2 stops of improved and usable ISO performance (low light capabilities)" compared to the MkIII to make me upgrade to a MkIV from my MkII.
Going from a MkII to a MkIII is not worth it IQ-wise.
On second thought maybe a 1D style body would be an upgrade ergonomically (haven't ever used one so can't really say). However for this reason a 1D series body is quite possibly the next step in my upgrade path although it really is a lot of money. I expect the 5D series to remain the more compact form factor it is now.
5D3, 5D2, Sony α6000, S90 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses