Re: DPMS pattern VS Armalite for build
.223 AR-15 rifles are more or less identical in terms of parts/interchangeability/etc. The only "oddities" would be things like some Colts having "large" pin triggers/hammers and larger pivot pins. Nothing you need to worry about with current production and nothing you need to worry re: Armalite vs. DPMS.
Now...any .308 AR is a WHOLE different ballgame. There are no "mil spec" standards for the .308 AR platform rifles so you'll have variations among different manufacturers which come down, in most respects, to "Armalite pattern" vs. "DPMS/KAC pattern" rifles.
The primary difference between the platforms is the magazine. Armalite uses their own mag setup and currently, there is no aftermarket source for their mags. The mag cost for the Armalite pattern is a little more expensive than the DPMS platform (at roughly $30/mag when buying in bulk like a 5-pack), but the Armalite mags are 100% reliable and built to last a long time even with field use/abuse. The DPMS pattern can use any number of mags, including DPMS factory, KAC factory, C-Products and Magpul LR20 PMAGs.
The other thing to be aware of is that only some parts/accessories are interchangeable between the platforms while others are proprietary to each platform (and both will use SOME AR-15 parts interchangeably such as triggers). For example, barrels/extensions (with some argument) and handguards differ with both platforms. Once you have chosen a platform, folks here can easily recommend sources for components, including receivers (U/L), barrels, BCG's, handguards/free-float tubes, triggers, etc.
Both are excellent semi-auto platforms to build on (I have used both extensively for personal builds and rifles I have put together for friends/colleagues)...just decide which you prefer.
Here's a little "light" reading for you on this topic:
http://www.ar15.com/forums/t_3_121/413443_.html
Good luck!