As a UA plat, I can offer an opinion here.

AA Plat > UA Plat.

Yes UA plat has free UDU but don't count on those much.

AA gives you 100% bonus miles as a Plat where UA Plat only gives you 75%. If you are UA gold, you get 50%.

Personally, I think AA treats you better though I can't really complain about how UA has treated me (other than some issues in a few lounges).

As an AA plat, I'd be happy to shell out for 500 mile upgrades. I tend to like their planes better as well (bigger planes aka less RJ's).