Not necessarily. I cut 5-6 acres/week with my ZTR. It takes me roughly 2 hrs/week, for lets say 6 months out of the year, so 48 hours per year. Those hours will add up on either machine (if I use a ZTR or a tractor) but the hours will add up more quickly on the tractor because it'll take more time. Regardless, Oil is Oil and Filters are Filters... you'd have to spend money for them for either machine. Tires, belts, all those things will wear out on both machines for the hours you're putting on them, but those wear items will usually be more costly on a tractor.
Now, if the OP didn't have ANYTHING yet (no mower or tractor) then the discussion would change I think. He could then decide on the debate of "1 machine or 2"... But since he already has a ZTR, and it's paid for, it seems the best option (unless money really factors in) is to keep it and get a tractor to handle the "Tractor" work he needs. He'll save money right off the purchase because he won't need a mid-PTO or mowing deck. That'll probably equal the money he'd get from selling the ZTR anyhow.