Don:
I'd be very cautious about using a ZTR on sidehills. At the very least ask the dealer to let you take a demo unit and try it where you're actually going to use it. I've used a 48" Scag Tiger Cub, both with and without the clamshell grass collector since '05 and to tell you the truth, I'm always puckered up when mowing the parts of my yard that are sloped. Regardless of what others have said, ZTRs are not ideal for sidehills because the weight is primarily on the two drive wheels, with the front wheels acting as castors. When you get on much of a sidehill, the rear tires will slide sidways and the front wheels will caster. I mowed the same yard for 20+ years with the 129 and with the weight distributed between the front and rear tires, never had a problem with it sliding. ( Why did I change to the Scag? - I had lots of things to mow around, not many big open areas and my creaky old elbows were getting tired of cranking the 129 steering wheel.)
Other comments - I'd never buy anything except a commercial grade unit. Check the thickness of the deck steel, get one built like a tank (the Scag I bought uses 1/4" + plate for the top of the deck). Get a unit with greasable pivot points (lots of zerks). Look at the drive system - if its got the one piece, unitized transaxle, skip it - they're not tough enough - look for separate pumps and hydraulic drive motors with the motor shafts actually the axle stubs. Make sure there's a good filter in the drive system hydraulics, too.
One thing nobody will tell you is that the primary feature of a ZTR, turning on a dime, is its biggest fault also. If your yard is soft, it'll tear it up every place that you do a "zero turn". You end up learning to do "J turns" when you're mowing an area that requires a lot of back and forth.
I'll admit that it's fast - I cut my mowing time in half - but for a hilly lot with lots of trimmimg, if I had to do it over, I'd buy a regular tractor with as tight a turning radius as possible, with power steering if it's available. Heresy for all the ZTR owners, I know,but I've never been real happy with the way the lawn looked with scars all over it. On big,dry, flat lawns like my neighbors (who both bought the same units I've got based on using mine) it's great. I love the way the Scag is put together, and with a hitch on it, it's damn near as useful as the 129 for utility work (park a full size two wheel utility trailer with a ZTR in a tight space once and you'll never want to hook it to a regular tractor again..). We built a hang off the rear bumper sprayer unit for the ZTRs and that works great, too.
Anyways, that's my story and I'm stickin to it...also, as Kraig is fond of saying, YMMV, yadda, yadda, yadda...
BTW - hello from sunny and HOT Florida, where there's no yard to mow (I knock it out in 5 minutes with a weed whacker) and the back roads are quiet and perfect for crusin' the Yammy (except for an occasional armadillo speed bump). I'm staying away from MI until every other snow bird has flown north, then maybe I'll follow.