I had a Simplicity Citation ZTR for few years, great cut, fast, and loved the controls. However, I do have slopes and it just got old after a while having to do the slopes with a trimming mower. If you have the right deck and blades then you can't beat the ZTR cut on flat property even if you have a lot of trees.
On the other hand, having the tractor has one main advantage for me, when I had the ZTR then I was the only driver and no one else was able to handle it in the family. Now it is different story with the tractor and get many more usages from it.