I would hope the ZD is more expensive, its easily twice as much tractor.
I would not buy a ZTR without doing a demo on some hills. Its hard to tell which ones will do well. Generaly we see Kubota out performing most of the stuff that we see because they are simply heavier and have larger tires. Thats not a set in stone rule though, some of the Ferris mowers do shockingly well even though they are smaller.