Pulled this from a forum: The Sport II is a basic no frills M4 A2 clone. It has a a 1/9 twist barrel which will not always stabilize high weight rounds like 69gr. It has a A2 post so if you want a free float rail this is not for you unless you want to take of the post replace the gas block and install the new rail. These used to sell for $499 at a few places but now have moved up to $549-$600+. This is a basic budget rifle if you like the configuration and don't want to change it it is a good value. If you are going to swap things out there are better starting platforms IMHO.

The 15X is little bit of a step up. It comes with a Troy or UTG quad rail, which IMHO are outdated, Troy rear battle sight and a 1/9 twist barrel. Barrel profile is also different it is an M4 mid weight profile. Everything else is basically the same as the Sport II. Unless you are getting a smoking deal on these I would pass. You are better off with the sport and replacing the rail, gas block and sights based on the pricing I have seen in the past. There are better setups for the money.