I was in your same boat. I decided it probably wasn't worth the extra $200, and bought the 6933 from Howard at JB Arms. All Factory Colt SBR's are roll marked M4LE. That goes for the SAW guns, the 6933, and the 6921 (Which I also decided to buy, instead of the 6721SP). Except for select fire law enforcement/military models, these are the only factory Colt SBR's I am aware of.
As far as differences, obviously a 10 or 11.5 inch barrell won't have the "step" cut for the 203. Other then that and the twist rate you referred to, I don't think there are any differences.