As always... Whatever fits your horse...
But seriously I'm not a fan of Stubben (sorry, SR!), and even less fan of AP. Stubben never felt good under my butt (plus all I tried were bad fit for either of my mares). Collegiates are nice saddles though.
Now if Stubben would be dressage I'd advice Stubben over Collegiate (as IMHO Stubben is a higher end), however with AP I found it to be rather hard to do a flat work (if you plan on doing it seriously), and every AP I tried gave me a chair seat. So I'd say try Collegiate.
Won't know what fits until one turns up, that is the joy of long distance shopping. I'm hoping that the Collegiate will be the right shape for him, and that the changeable gullet will help to get a good fit.
I actually prefer to ride in an AP rather than a dressage, but to be fair the only dressage saddle I have I mainly rode Fancy in, and I think it was her that was the issue, not the saddle