There are two main patterns of .308 AR-pattern rifles. The Armalite and DPMS/KAC.
The Armalite was marketed during the now lapsed assault weapons ban. Their strategy at the time was to make it compatible with widely available M-14 magazines, so you could have increased capacity even with the ban in place. They have a diagonal cut in the rear joint where the lower and upper meet, making them incompatible with the other pattern.
The DPMS/KAC pattern is more widely used, because the military adopted this pattern for its new DMR/sniping systems, and a lot of the accessory and mod market followed it. As stated above, the popularity of Pmags put enough pressure on DPMS to make them change to use them as well, and honestly I will be surprised if they don't also release a new version of their rifles that are compatible with the DPMS pattern.
Most of the lower parts, including the trigger, are the same as the AR-15. If you have a rifle chambered in .308, you can also use uppers in .338 Federal, 7mm08, 260 Rem, and .243. Same shell casing, magazine, and bolt face. (I wish someone would make one in 22-250, but apparently the casing taper is too extreme for reliable feeding.) The Armalites may well be great rifles, but right now, the market has followed the other pattern. You can buy a stripped lower for a few hundred dollars and build it out at your convenience.
The other downside I am finding, is that anything made for .308 rifles commands a much higher price, for no other reason I can discern than they can get it.