lowers vary depending on how they were made -- milled or cast.
milled, as it's name indicates, is milled from a solid block of aluminum (tool marks can be found on lower), and are the higher end lower. "cast" is as it indicates also. pretty much, the castings (excess aluminum from tool work) from the milled lower are gathered (not really), melted down, and poured into a casting (mold), allowed to cool and then removed from mold.
it just depends on what you like. BEWARE! THERE IS CRAP OUT THERE! it's rumored though, that there are only 4 or 5 lower receiver manufacturers out there that supply for ALL the retail companies.............they just put their logos and names on them.
do a ton of research and quality reading, get a good quality upper and barrel, and BCG (bolt carrier group - bolt and bolt carrier), spend a few extra $$ on it, and you won't be disappointed. cuz once you shoot it (and maybe even before), it's yours! you're stuck with it, however much you paid for it..............if it's crap and you have to buy another one, then you're out the cost of it, PLUS the cost of the one that's gonna do you right.
"they may get me in a rush, but not before i turn your head into a canoe....." -Kurt Russell (Wyatt Earp) in Tombstone
"if it was up to me, i'd like to see, this country run,
the way it used to be, the way it oughta be, just like it's done, OUT HERE.........WAY OUT HERE." -Josh Thompson, country music singer http://www.youtube.com/watch?v=D0sYnro_3Rc
PEOPLE TEND TO ACT LIKE SHEEP, BOY I LOVE MUTTON! -Me