I used a 3203 a few years ago for brush hogging a pasture and found it incredibly irritating to not have cruise control. I mention this because if I were the property owner, I'd have opted for a 790 in a second. It struck me as a better built machine and having the gear drive would have meant that I'd have a consistent speed and would have been comfortable the whole time. Instead, I found myself bouncing around a rough pasture on a hydrostatic tractor that lacked cruise control - so, with each bump, my foot slid off the pedal and I would slow down, get back up to speed, hit a bump, slow down, get back up to speed, etc...
My point?
If I were doing a lot of loader work or mowing a yard, I'd say hydrostatic has the advantage. If I were mowing a pasture with a brush hog mower, I'd prefer gear drive. Of course, if money were no object, the 3320 does have cruise control... but I'd be leery of a 415 hour rental tractor.
Perhaps you could tell us what it is you tend to do with the tractor most often. Maybe the 3320 makes more sense, but given that it has only 5 more engine
hp, I don't see any immediate advantages for the extra $6500.