I dont usually post on alot of these threads for higher end mowers... but I think the reason the tractor spun a wheel was because it did not have a locking differential. Where as ZTR's have 2 wheel motors which obviates the need for the diff. By the way if your hill is 15% then mine must be way more... I can mow across it with no issues, so I have to believe yours would be a piece of cake. Any congrats on the new mower... it looks like a heck of a mower.