I've had status of varying levels with both airlines, and in fact still do...Gold with UA and Executive Platinum with American. I find American to be way better for Executive Platinums than 1Ks with United. At the lower elite levels, I think it is somewhat of a wash, but you are indeed more likely to have an E+ seat on United than with AA given the relative size of each E+ section. I find UA to be a much better experience in Y than AA's miserable Oasis seats, too. Just realize as a Silver or Gold out of ORD with UA, upgrades are pretty much non-existent on most routes. I also find UA's MP program to be worse for redemptions that AA's as of late (words I never thought I would utter) as MP got Kirby'd in a big way during the pandemic. I also find UA's ORD terminals to be much more chaotic than AA's ORD terminals, but YMMV.