Pros and cons of each in my opinion
Pre-built
Pros
Warranty
Higher resale value
All parts from the same manufacturer and assembled by the manufacturer
Usually cheaper
Gas system typically balanced well with the recoil system
Cons
Usually not exactly what you wanted so you spend some more changing out parts then having a box of parts you don't want or having to sell those parts.
Typically mid grade barrels that are not properly heat treated and stress relieved so you get stringing as it warms up.
Piece together
Pros
Everything you want from day one
High quality parts where you want them and economy parts where it matters less.
Higher quality barrel (if you choose to spend more money)
Experience putting it together which helps in troubleshooting if needed
Cons
Usually costs more because you have to talk yourself out of buying a $500 barrel and the latest greatest super duper lightweight parts.
No warranty
If you're not overly experienced you could end up with gas system/recoil system balance issues leading to harsh cycling or failures
Have to purchase some extra tools if you want to do it right.
Lower resale and harder to sell a FrankenAR
I'm sure I could come up with more for all of the above but that's what comes to kind right now.
I love building AR's but I don't do it to save money.
For those who have built their ARs, are there any brands of uppers you'd suggest? Or pretty much any reputable one will do? I am building an AR15, the lower is done, now looking for compete upper as I don't want to tinker with assembly of barrel at this stage. thanks!
If you're buying US made forged receivers then any will do, they're all built by three different forges then sent out to the manufacturers for finishing and branding.
Stay away from NEA/BCL and Norinco and you should have no issues.
I've never heard of anyone having a problem with AERO from Saskatoon Gunworks and their prices are good but there are many others that are just as good.
Good luck