Hitler would probably be less difficult to be around since he was a non-smoker. Stalin was always constantly smoking that real shitty Georgian tobacco out of a pipe, and he liked dump the ashes out on top of the heads of companions who fell out of favor with him.
Neither. Both would kill me. Either I'd be useless to them and get killed for that, or I'd be useful to them for a short period of time, like Ernst Rohm or some shit, and then get killed to improve the image of the party, or that paranoid faggot Stalin would decide I was a threat to him and kill me. Either way I'd end up dead.
So either choice I had, I'd either shoot the fucker or more likely run away.