For complete ease, simplicity and reliability, the complete upper is the way to go.
Bolts and barrel extensions do wear into each other. While you can get away with swapping one between two uppers, the more you shoot the less it is recommended.
Bolt carriers and upper recievers will also wear into each other, but the contact surfaces are flat and sliding, not complex and sliding/rotating. Swapping a carrier back and forth between uppers is not too big a deal.
Now, for the practical side of it.... unless you will only shoot your rifle very little, I'd recommend against sharing the bolt. Between the reduced wear of not swapping parts, one can back up the other as an emergency spare. The carrier isn't very expensive, and it is mildly annoying and somewhat dirty to swap. With the complete bolt/carrier assembly only making about $100-$120 difference when buying a complete upper, I would get it. Besides, it is always easier to complain to a manufacturer about a misbehaving product if the entire assembly is theirs. (on the off chance you have a problem)
Brands....
Bushmaster,
Armalite,
Rock River,
DPMS are all good. You can get a decent to good deal on some DPMS and Rock River products through
CMMG - I've talked to them and bought from them before - good people.
Be aware of what you want and what you've looking at. Comparing one vendor's stainless match barrel with another's unlined plain steel barrel or chrome lined rough duty barrel without understanding the differences may push you to a "less than perfect" purchase decision.