So far I mostly fly United because of good nonstop connections and prices out of SFO, and because I don't fly enough to have a chance of FF status on multiple airlines.
Right now AA still awards miles based on distance, but once that ends the second half of the year, you'll accrue miles based on the ticket price less taxes, just like UA. One distinct difference is that AA will not put every elite on the upgrade list like UA, so if you're clearing a lot of domestic upgrades at UA at midtier you might not see those at AA (you'd need to purchase upgrades beyond what you'd normally accrue - no free upgrades).
Look at Alaska which partners with both AA and Delta. They're still awarding miles based on distance, though anyone's guess how much longer that will last. Probably safe for this year. Downside here is that there are no reciprocal upgrades with AA or Delta, so you'll be riding in coach as an Alaska flyer. AA does offer Alaska elites some perks such as early boarding and seat selection. Don't know if Delta extends any perks.