Hmmm....a Sony A7 mark 2 user here. IMO, don't really go into the lens mount debate. I'm using Sony lenses, specially the awesome 85/1.8 and it is just too good for the price. Most of the Sigma lenses are also available for E-mount now. There is Tokina (and Tamron) with excellent lenses....and the Chinese and Korean lenses are good too. With current generation Sony/Nikon/Canon, camera is rarely a limitation- it is the user who is detrimental to the performance. FF or APS-C ? Hmm, Sigma has some nice cheap APS-C lenses which can be used for "pro" work. But a safer bet is A7iii.