https://www.united.com/web/en-US/con...onomy-faq.aspx try there.

The TLDR is seat selection is a no, normal carry on baggage yes and a free checked bag, you receive award miles 50% but not tier miles. United Club access too.

On AA seat selection would be at T-24, normal carry on and free checked bag (is it two for golds?). club access. not sure on mileage earnings.

Looks like the only difference is a small chance of avoiding a middle seat on AA and a very small amount of miles as at most you will be earning a tiny amount of TPs and 200 or so miles - book schedule and price I would argue in this case. it's short enough to survive in a middle seat as you won't be able to get an extra legroom seat anyway.