I also have both a HST and a GST model Kubota. Smaller than what you are considering at 21 hp and 39 hp. It took a couple of days with the GST but it becomes pretty natural shortly. As far as missing the HST, I think it is easier to operate but not enough to under buy if the tractor I needed was only offered in GST. That was my situation at the time I bought the second one.
MarkV