I've owned the Fenn XT in fiberglass, and have a couple of friends who own the V10 Sport Ultras. To sum it up in a nutshell, the XT is more stable and slower, while the V10 Sport is more tender, worlds lighter in the Ultra incarnation, and faster. That said, the XT is a wonderful rough water boat and really comes into its own on textured water. Great at picking up little bumps downwind. My fg version always felt sluggish to me though and the seat rubbed me raw. The Sport is built for big guys and many pad the seatwell in. In terms of stability most novices climb on the XT and wobble away reasonably dry while on the Sport they swim fairly quickly. The Ultra boats are very fragile while the XT in the fg was a tank-shrugged off darn near everything. In terms of rough water speed, they're probably fairly close with the advantage Epic. The Epic feels faster though, which counts toward the 'fun to paddle' factor. Both really nice boats, but maybe also try a Huki S1-R, which I have now-it has the best qualities of both.
"Nice? It's the only thing," said the Water Rat, solemnly, as he leaned forward for his stroke. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats."
'The Wind in the Willows'~Kenneth Grahame