My local dealer is an ex council subby, meaning they used to do 100's of acres every week. He used to run 72" Iseki's but went to 60" Scag TT's.
I was only talking about this with him the other day. He would steadfastly recommend going with the 60"! Believe me, if anyone knows about this sort of thing, its this guy!
A dead set wealth of info whom I hold in the highest regard!
Looks like a good opportunity for a demo. Get 2 of the same machine but with 60 and 72" decks and see how they go! In short grass the 72 would probably win but put it into some long stuff and it would be a different matter I bet!