I'm certainly not an expert, but my impression is that one type of lower can be made into a selective-fire (full automatic) M-16/M-4 clone with the right M-16/M-4 parts (and the right license/permit to do it legally) and that the other type could not be made into an M-16 clone (but could be rigged to be made into a selective-fire rifle with a certain product, again with the proper license/permit for legal purposes). My impression is that either type should work fine for a standard semi-auto AR-15 build.
You might post in the AR-15 forum for a more definitive answer.
