If the only job is to cut .5 acre of 20-25 degree hillside with a pond at the bottom, I'd lean towards the trying the remote controlled mower at less than $3,000 because going from 42" down to 20" of width of cut on a .5 acre isn't terrible if you can avoid having to ride the mower yourself.
Another option I have considered for my slopes is to buy one of those 5' trail mowers that have four wheels to winch up and down the slope, but the irregular ground contours and lack of steering control keep me from trying it.