I also vote for the 120mm lens - it's a very,very sharp lens and you'll have to get very, very close with the 80mm lens (too close for my taste).
Being that close to a subject is very uncomfortable - personally I also like to use the 210mm with extension tubes but the quality of the 120mm lens is way superior.
And you don't have to get the newest shiniest lens, as far as I know the optics haven't changed much and on a 'lower' resolution back there is absolutely no difference between the latest PhaseOne macro and the 'old' manual focus Mamiya lens for AF.
However I wouldn't recommend using the really old completely manual 'M' lenses, it's just not comfortable to work with in the studio, especially not when you have to focus a lot (you'd have to open and close the aperture all the time...really annoying).