I probably lean towards black too, from a visual point of view, though I think the chrome is harder wearing (not that it matters). Actually it might matter in one respect - because the paint wears off in a way that the chrome doesn't, it is easier to see if a black one has been well-used. Whether that is a good or a bad thing is less clear....a cosmetically mint camera often hides 'issues'.
This next point doesn't apply to the FM series, but on bodies with plastic top/bottom plates, they definitely look and feel better (and a bit less plasticky) in black. Things like the FG, FG-20, EM (though this only comes in black), and the FA (top plate plastic, bottom plate brass, for some reason). Probably because the chrome isn't really chrome, and you can tell.
I think generally black versions of all Nikon cameras carry a price premium though, so if you can't decide which you like more I would recommend going for chrome, as you'll save a few quid.