This is my first build. I've completed my lower and I am sourcing parts for my upper. Does it make sense to match the finish of the BCG and Barrel? I don't care about the cosmetics. What I'm thinking about is wear on one or the other because of any differences between the hardness of the two finishes. DLC vs. phosphate, that sort of thing. Does it matter?

no, not an issue

This ^^^

Much appreciated, thanks.

Where can I get  a low mass BCG 556/M16 Nitride IN STOCK anywhere in the US?  
Brownells, RCA, or Durabolt ?

BCG never touches the barrel

lol!

No.  

Select barrel and bcg for the quality and properties you deem important.  Get the barrel that meets your needs.  Do the same for the bcg.  I run phosphate, nitride and nicker boron bcgs, each for a different purpose.  That choice is independent of barrel style construction.  

Your bcg runs in an aluminum upper with only the bolt lugs contacting the barrel extension.  Differences in steel on those contact points is irrelevant, provided the bolt is of proper mil spec substrate steel construction.