Most all receivers are forged by Maremont Forge, different companies just have their logo stamped into the forging during the mfg. process. Then every company weather it be X co. Y co. or Z co. has it machined to Mil. specs. They are therefore all the same with the exception of the name on the side. The difference is in customer service and warranty if by rare chance there is a problem.
Forged is much stronger than cast, cast is cheap in both price and strength. do not buy cast unless it is as a gift to someone you do not like.
The difference between .223 and 5.56 is negligable. The 5.56 chamber is a tiny bit larger to accomodate the NATO ammunition, whereas the .223 is a tighter chamber, but there is really no difference that should matter unless shooting benchrest or competition at 800-1000 yds. Do not worry about it, both will interchange and fire just fine in a 5.56 chamber, plus you can use the cheap surplus ammo for practice.