I'm far from an expert, but based on my own personal (limited) experience you do not want a carbon fiber AR-15. I just cannot see how one could possibly stand up to the rigors that an AR-15 will likely be put through in the near future.
If you look at gun and ammo sales, this is a country that's (IMHO) preparing for civil war. Do you want something tried, true and reliable or lighter weight?
I'll go for reliable any day.
BTW: I HIGHLY recommend The Rock River Arms LAR-15 Series. A finer, more reliable AR-15 you won't find.
Disclaimer: NO, I DO NOT work for Rock River Arms. I am however one of their customers and a fan of their quality craftsmanship.
I bought a Core15 AR, but haven’t been able to shoot it yet.
Lost all mine in a tragic asteroid strike recently.