We have a Kubota ZD21 (21hp diesel, 60" deck) and a Grasshopper 614 (14hp B&S Vangaurd, 48" deck) and 725K (25hp Kohler, 60" deck) . The Grasshopper 725K is by far my favorite one to operate.
Kubota ZD21:
Pros: Excellent Kubota diesel engine, nice hydraulically controlled PTO and deck lift, good commercial grade machine, no repairs in 600hrs
Cons: Rides rough as an ox cart, the hydro drive has marginal power on steep hills (it does have fluid in the tires which makes it heavier)
Grasshoppers:
Pros: Ride is the best of any zero turn mower I've run, especially the 725K (wide rear axle), very heavy construction, built for longevity (lots of grease fittings), G2 drive system in the 725K has plenty of power for hills. The older 614 has been an excellent mower as well, it's a 1992 vintage with nearly 2,000hrs on it and basically no repairs.
Cons: Traction isn't great on hills with turf tires (both have lug tires now), electric PTO clutch and manual deck lift.
I'd go with the Grasshopper for your situation. They are an excellent machine all around and not many people seem to know about them any more with popularity of the mid-deck ZTR's. Grasshopper has been building zero turn mowers decades before most of the other zero turn companies were even in existence (since 1969.) With the out front deck, lug tires and wide rear axle they are very stable, capable and extremely smooth riding. The Kubota models are very good machines as well, we've had zero trouble out of ours and it's very well built, just rides really rough.