Wouldn't be easier to just ask in circumstances where the longer buffer tube actually works out, what's the difference?
The difference is that one is a great design with a great track record, and the other is just the other.
The ArmaLite recoil system works very well with DPMS. The distinction is that ArmaLite did it first, and very well, which is well demonstrated by the fact that their recoil system will work well in a DPMS.