Hi
It all comes down to preference, I have ran both ZTR's, traditional Garden tractors and Front mount mowers.
From running ZTR's: I work for a local conservation authority as well as run my own part-time lawn care business. The conservation authority has a Deere 997 Z-track and a Kubota ZD28( Kubota suppose to be traded this spring). On hills, the Kubota was lower to the ground and more stable on hillsides then the Deere, but I found the Deere 997 Z-track to really cut grass in a hurry on a flat ground which does a nice job. You get into wet conditions though, and they can get stuck easy. We have lots of weeping beds that we cut in the park and with so much grass we are cutting rain or shine in prime growing seasons. If you run a ZTR properly they won't cut up the ground.
AWS: I currently have 2 AWS mowers in my fleet, a 455AWS with 60" deck and a 2007 Deere X744 AWS with 48" deck. I went with AWS, as I can do more with a tractor then I can a ZTR.... I am thinking of putting a Rotary broom on one for power sweeping. The AWS's Deere do a nice job cutting grass as well and are very versalitle machines but when repairs are needed on the AWS, it can be pricey. You do get 4X4 on a Deere X749 AWS that would really help stablity on the hills as well.
I still like steering with a steering wheel, so I went AWS instead of ZTR.
But it all comes down to preference.
I have a hydrolic Dump MCS on the back of my JD 455AWS and is a really slick machine for leave pickup in the fall, another reason I went AWS. I did look at a Walker ZTR( which are Awsome) but the closest dealer was 50 minutes away vs the Deere dealer which was 10 minutes away. That and the JD dealer offered great service and support.
Its all up to you, but I prefer AWS my self for my applications, but that does not mean its the same for your plans with it. There are some nice ZTRs out there as well, that would suit your needs as well. My suggestion would be to demo both, a ZTR vs an AWS and see which one you like better.. If you like the ZTR go ZTR, you like the AWS go with AWS.
A Grasshopper or Walker ZTR would work well, because you can put other attachments on them, like a blade, snowblower, broom etc... plus if you got a lot of trees, the MCS on the Walker is nice to have as well. I looked seriously at a few Walkers, but they were expensive.
Take Care Jason B