Eighteen inch barrel? vs. a sixteen inch barrel? Is the little bit of extra velocity worth it? With good barrels I wouldn't think you'd see a difference in accuracy.
Anymore, I get just about all my AR15 stuff from PSA (Palmetto State Armory).
This 18" upper:
http://palmettostatearmory.com/psa-ar15-18-rifle-length-223-wylde-1-7-stainless-15-keymod-upper-no-bcg-or-charging-handle.htmlalong with the bolt/charging handle, scope, scope mount, etc. became this rifle. It's a good shooting rifle. It has a different scope mount on it now so I could get the scope farther forward (gotta have my nose on the charging handle whether using the optic or the back up sights. Oh, I got the A1 fixed stock of ebay. I'm finding I like those better than the carbine stocks and much better than the A2 fixed stocks.
The latest stuff from PSA now looks like this. It was this upper from PSA with the rest of the parts.
http://palmettostatearmory.com/psa-16-5-56-1-7-midlength-nitride-lightweight-keymod-freedom-upper-w-bcg-ch-mbus-sight-set.htmlSo far I've not had to send anything back to PSA or call them for advice. On this last one (the 16" I thought I might, but the problem with not chambering seemed to clear itself but I never did see/find out why.)
Good luck, with whatever you get.
Everyone has their likes/dislikes.
I like a keymod free float tube (free float tube to insure the barrel stays isolated from accuracy robbing contact/stress). Keymod because that's what all mine are.
I like a scope over a dot sight (now a scope with a dot is sort of okay) because I can shoot smaller groups and ID targets better with a scope than with a 1X red dot optic (yeah, I know the 16" in the picture has an Aimpoint on it but it will get a scope sooner or later, all of them do.
I like a chrome or NiB bolt. There's just no comparison in how fast a "slick" bolt cleans up vs. a standard parkerized bolt/carrier.
I like a nice trigger. I've got RRA triggers and one Giselle and either are nice.
I like GI web slings. I've got a couple leather slings on old bolt action rifles (GI bolt action) and they are just tough to use on a hot sweaty day. The wet skin isn't slick to a leather sling, it makes them stick like glue.
I've got PMags and GI aluminum mags and even a few steel mags. Every brand of magazine I've bought works fine except CProducts mags. The black stainless CProducts mags. for the 5.45X39 S&W M&P 15 work great. The aluminum CProducts mags. for the 5.56X45 AR15's gave me a 30% failure rate. Both batches I ordered. Wouldn't feed the last 2 rounds out of the magazine. First 28 were great, the last two would nose down jam. Changing springs didn't help.