For $800 or a little more you can do better than those two.
I agree that PSA has some great prices. Like Jason I recently bought their milspec receiver extension kit (with buffer, spring, end plate and castle nut - no slider) and the quality was really good and for only $40.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek