In reality, I was planning to open a thread in the OW forum, but first I wanted to gather some information on the two programs. When laying down the info, I was surprised to see that for my flights on BA it wouldn't make any difference (besides the upgrade priority) if I were a BA silver (vs. AA Plat). In UA it makes a huge difference if you are a UA elite vs. *A elite (E+, comp. upgrades, treatment, etc), but I guess that's a totally different world.