The more expensive on is the 6920 SOCOM. Mid weight barrel vs gov profile, ambi safety, matech buis(these are $120 retail) and KAC RAS($400 retail).
It was a SOCOM version of the M4 the Army used, but now the Army is moving to all M4A1's. the only diff is the civilian ones have a 16" barrel and S-1 FCG only.
The SOCOM is actually a good deal if you take into account the price of a new RAS and Matech which would be $520 retail for both. That means the base rifle is around $900.