No real disadvantages or differences, other than the fact that a factory M16 would command a steeper premium because it is a factory gun and not a piece converted by some other person or manufacturer. The other obvious difference is the mag fence. Most, if not all, SP1's were slabsides, but many of the conversion guns on AR15's (later model Colt's, Sendra's, Oly's, etc) as well as the later Colt factory M16's have the raised mag fence. But for a shooter, there's no real difference.