Since this is a RZT thread I'm assuming you're not looking at or need a commercial unit with bigger tires, possible suspension, or front mount....so
Trying to keep it short: $ for $ will often put larger tires on a GT than a ZTR, this, in many yard conditions, will give the GT a slight edge, but mph to mph I don't think the difference it huge. Another thread also discussed the seat location and how the pivot effects the ride, but again <font color="red">I</font> don't find the effect huge.
That being said, the manuverability you have with a ZTR may "encourage" you to mow faster and take a beating you wouldn't get from a GT. /forums/images/graemlins/grin.gif
All boils down to [IMHO] if you mow the same speeds, the ride difference isn't huge, but you'll still save plenty time in most situations.