The Diploma does have a very deep seat (the saddle design is very different than most of thier saddles, and this is the only Collegiate that I have ridden in that I actually liked for the exact same reason as your trainer), but even with that deep seat it has a very close contacty feel. It helped me sit my old Quarter horses harsh canter, and made it feel like I was floating on a cloud. Yet when jumping and cross country came to play there was no "locking in" I usually feel in saddles with a deep seat. I was free to move how I pleased, and my trainer said it seemed to help my jumping ability. But MIEventer is absolutely correct. That is my view on things, and it may be different for you. I still highly recomend that you try the Diploma, but you should also try the wintec along with other saddles that may catch your intrest.
"Riding is not Equitation... The moment you believe it is, is the moment you quit riding."