I wanted to update everyone. I decided to go with a Grasshopper 623t with 52" deck, dual rear wheels, and WWF filled bar tires (45 lbs each). Price with 0% financing (48 months) and tax was 10.2 k. Paying cash would have only saved me $300, so my 10k will make a lot more in a mutual fund over the next four years than it would have if I paid cash for the Grasshopper.
I wanted to write down some thoughts as far as hillside stability and slope holding stability since nobody online seems to quantify slope angles when discussing hill performance, they simply say "steep," "really steep," or pull a number out of their butt (30 degrees is a lot steeper than most people think it is). All of the angles I will mention have been verified with an inclinometer resting on an eight foot long 2x4.
Firstly, I could see the Grasshopper front mount mowers being horrible or excellent slope mowers depending on whether or not they are configured correctly. There are multiple mounting points to connect the deck to the drive unit, which control how much weight is transferred to the drive wheels. There are turf tires or bar tires. There are single tail wheels or dual tail wheels. If you had the deck set up for minimum weight transfer and had turf tires with a single tail wheel then you would be slope limited. My grasshopper is setup with bar tires (filled with WWF), max weight transfer, and dual tail wheels.
The Grasshopper's most traction limited direction is going straight uphill. The weight on the drive wheels is reduced when going uphill and increased when going downhill, which is the exact opposite of a mid mount ZTR or lawn tractor. That being said, uphill traction ain't bad. It's pretty much equivalent to my Crafstman LT with open rear diff and wheel weights. That is to say, slippage will occur around 20-25 degrees depending on turf conditions (assuming the ground is dry).
Backing uphill is a different story -- it'll back up hills 25 degrees no problem (I haven't tried steeper but it may do it). Personally I prefer to have more traction going downhill, as that's when you need it for braking or backing out of a corner.
Side hill stability is excellent, I've had no issue mowing cross slope on slopes of 15-20 degrees. More may be possible, but this machine is new to me so I haven't explored it's limits yet. You will tear up the yard if you expect major traction from the uphill wheel, but the uphill wheel will have enough traction to keep you pointed perpendicular to the slope.
The ride on the Grasshopper is un-freaking-believably fantastic. I mowed the roughest most rutted section of my yard at top speed (8.5 mph) and never felt a strong shock through the seat. This contrasts with my Craftsman LT, which will knock your fillings loose if you tried to mow the same piece of land at its top speed (about 4.5 mph). The ride is really that's good, it's the difference between a tightly sprung car with a short wheelbase and a huge 1970s Cadillac with soft suspension.
I demoed a Ventrac 3200 series in addition to the Grasshopper. It was a nice machine with unmatched traction and good hillside stability, but its mowing ability and ride comfort are a fair bit behind the Grasshopper. That's not necessarily unexpected -- the Ventrac offers some tractor capability that the Grasshopper does not (e.g. a slip scoop), so it's a different type of machine. The grasshopper does offer some other attachments (e.g. a snow blower and a dozer blade), but you can tell that its primary purpose is mowing.
That's all I can think of for now, please let me know if you have any questions and I'll try my best to answer them. I never thought I'd say this, but I wish I would have spent 10k on this mower five years ago when I bought this property.