I built my AR out of parts I bought (mostly) from
Rainier Arms™ | Specialist in High End AR15s/M16s,Stocks,Grips,Parts,Accessories & much more. They were top notch and I enjoyed the build process.
It's an 18" White Oak SPR barrel in a VLTOR VIS 12" upper plus various other parts.
One thing I do recommend is getting the longest gas system you can. That means if you get a 16" barrel, get a mid-length gas system. An 18" barrel lets you get a rifle length gas system and it will be more reliable with less recoil.
I also have a Robinson XCR and to be honest, I like the XCR better than the AR.