It seems like the twins use a bit more gas than the one-lung, but I'm sure that varies by machine. I just know that our 19HP Kawi twin uses almost TWICE as much fuel as our 15HP Kohler ( Yes, iI know there is a size/Hp difference, but TWICE the consumption?). Come to think of it, the 19 Kawi uses about the same as our 27 Kawi twin ZTR (if not a lil' more). That being said, mechanically, the single cylinders obviously have a few less moving parts if you're worried about breakdowns/repair. However the twins usually seem to run much smoother, as far as operator experience (vibration/whatnot), and they seem to, at least with our machines, start easier. I would guess it depends on user preference and what your wallet is telling you. All of my comments aside, personally, I'd still go with the twin.
If you want to have something you've never had, you need to do something you've never done.