The Ultimate Sporter comes with the Air Arms Q-tech moderator, has a slightly shorter barrel (19.45") and a smaller air cylinder (216 CC) giving a smaller shot count (about 20) while the XTRA FAC has a 19.5" barrel, 231 CC air cylinder and can give around 30 shots.
In terms of the stocks, the Ultimate Sporter has more adjustability than the XTRA FAC to allow for a better fit without having to do a lot of modifications. The Ultimate Sporter has more of a pistol grip while the XTRA FAC has a more traditional grip/feel to it. Another point to consider with the stock differences is that the comb on the XTRA FAC is more blade like and not as comfortable as some shooters like. Personally, I don't mind it, but others that have tried mine have complained and feel it should be sanded down a bit.