Can't you upgrade your Vega 56 into a 64 with a BIOS flashing? In that case, you should benchmark the nvidia against the Vega 64...
According to whattomine their are pretty much the same in terms of earnings..
Vega is outstanding at Cryptonight, very good at Ethash, and piss-poor at pretty much everything else. If you plan to do any Equihash, Neoscrypt or, especially, Lyra2v2 mining then get the 1080.
I really don't know which one i should buy.. According to whattomine i should instantly take vega56, according to nicehash gtx 1080.
I've read of troubles about vega56. But i don't know if that's a standard thing for each card or not.
I'm confused
