Man, you're not just comparing apples to oranges, you're comparing beans to cornbread, too!
A 4wd 4ws garden tractor compared to a tricked out zero-turn mower. They're not to the same animal at all. If, as you say, all you want it for is to mow, then some would say get the ZTR. However, I personally do not like driving a ZTR. I much prefer wheel steering to lever control.
Even with a barn full of tractors, I would personally go with another tractor, be it green or orange, over a ZTR. Even though you say that all you want it for is to mow, once you have a garden tractor, and you see how maneuverable it is up close to the house, I bet you'll find other jobs for it.
Be thoroughly convinced that you love lever driving before you make that decision SWMBO says has to last 5 years.