I agree with fsted2a, the plastic upper worries me more. Will it stand up to supporting the barrel over thousands of rounds. Seems like a lot of stress at the front of the receiver supporting a long front heavy rod of steel. I'd go with 7075-T6 alloy for both receivers.
Stag vs. Delton? Hard to say. Why not tell us which models and specs and price range. Got links to the versions you're looking at? Or better yet, look at PSA if we're talking $800 or so.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek