I have and like both.
Got a chrome M6 classic because it was a good deal, it looked practically new and I couldn't pass it up. Looks great with my silver biogon 35mm f2.8, though ideally I would like a chrome summicron asph on it but mainly for ergonomics and aesthetics. Waiting on a chrome rapidwinder II that will really top it off.
My black M6 classic is the 0.85, which is only in black I believe. This will be nice with my 50mm summicron when it gets back from an overhaul.
Both are nice, black is a little more discrete, and if i had to choose one it may be black, but chrome is super nice too. Never understood those who dislike worn black chrome, I think its fine....and more importantly, none of this matters for taking pictures.