As previously written, only a handful of companies actually make lowers, and the only real difference in most is in the engraving and the anodizing/finish.
I just put together a Double Star lower with a Rock River Arms parts kit, MIAD grip, and Vltor E-mod stock; everything went together fine. I (temporarily) swapped in my Bushmaster upper - no problem.
Why Double Star? It seemed well finished - nice, uniform, black - and was cheaper than other uppers at a recent gun show.