An in-spec forged aluminum lower is an in-spec forged aluminum lower. You're not going to see much difference so long as all the holes are drilled in the right locations. On the other hand, when buying a complete lower there are certainly differences when it comes to the quality of the small parts.
BCM rejects lots of parts and components as a regular part of business. Their goal is to deliver the highest quality product at the most reasonable price point that they possible can, but the former takes priority over the latter. On the other hand, PSA either chooses to sells their rejects, or they simply don't have the QA/QC in place to catch them before going out. That part simply cannot go unnoticed.
When it comes to uppers, PSA has historically had serious problems in the assembly aspect or production. I happened to receive one,but when I shared it here on THR, it went over like a fart in church. The more complex an upper is, the less I trust PSA's ability to build it properly. For a basic upper with an FSP and standard hand guards, it's kinda hard to mess that up. Introduce low profile gas blocks and handguards that require attention to detail upon installation, and you're on shaky ground with PSA.
Over the years of buying and/or encountering products from both companies, I've come to trust BCM much more than PSA. That's not to say that I wouldn't trust a "premium" PSA product, but I'm certainly going to go over it with the proverbial fine toothed comb before doing so.