No real difference to speak of. The receivers are identical, but the parts from one maker or another can differ, particularly the flash hider, it's configuration and how it is attached to the barrel.
Some ppl will claim the Norc or Poly will have better or worse internals, but I now think this comes in waves and either make can be as good as the other depending on the particular rifle.
Frankly though, I buy these for the receiver - the rest is just stuff to sell on the EE. The base rifle works fine, but I'm a USGI snob - no good reason other than "I can".