I am going to build my own and was thinking of buying two stripped lowers. One forged and the other cast. The cast of course would be the plinker, but I don't know much about cast and wondered if it is not worth just the 30-40 dollars more for a forged? Opinions and experiences would be appreciated.

I know an employee of Oly Arms who says that cast break all the time when they are broaching the mag wells.  This isn't a good indication.  I have no personal experience with these lowers.

I'm sure you'll now get to hear all sorts of horror stories about castings.  Since the recoil isn't directed into the lower, it's not much of an issue.  Now for a serious SHTF/Tac rifle type, I'd get a forged lower, just in case.  But for plinking, etc., shouldn't be an issue.  

I HAVE A CAST AR15 LOWER ON MY 50 CAL BMG,SEE MY WEB PAGE UNDER GUNS. IT HOLDS UP WITH  NO CRACKS OR ANYTHING THE GUN WEIGHT IS 46 LBS, ( NOTE I ALSO SELL THE 80% CAST ALUMINUM LOWERS ) AND THE MAG WELL'S ARE PRECASTED IN, NOT BROACHED( THEY MIGHT BE BROACHED AFTER THE FACT TO CLEAN THEM UP AND TO MAKE THEM TO SPEC
(ALSO A WELL KNOWN MFG DID A STRESS TEST ON A 356 T6 CAST ALUMINUM AND IT PULLED APART AT 46,000 PSI) THAT IS PRETTY STRONG
( I HAVE SOLD OVER 10,000 80% CASTINGS AND HAVE HAD NO ONE SEND ME ANY BACK THAT  WERE BROKE FROM "NORMAL USE"( IF IT WAS CRACKED IT WAS A CASTING FLAW AND NOT OPERATOR FAILURE OR MISUSE OF A FIREARM

JUST MY .02 WORTH

WILLIAM