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, G16 | EF: SY14/2.8, V20/3.5, 24/2.8, 35/2, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L, T28-300 Di VC PZD | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS, Metabones SB | Vintage FD & FL glass.