United uses UDU (upgrades if available--check on the UA board how well the upgrades are out of your home airport-BOS). UA doesn't use 500 milers or stickers for quite some time.

AA makes you pay for the upgrade with stickers (either from your AA account or buy them).

I would decide which airline works better for you. You may be able to request upgrades on all routes on UA but you will have Global Services & 1K's ahead of you. There is no upgrade currency as a Prem Ex. (I haven't been a Prem Ex since 2005 so I am not up to date on UA).

On AA, the upgrades cost "stickers" (500 milers--either in your account or to purchase). There may be a higher chance to get the upgrade since after the Exec Plats are upgraded since it cost stickers for Plats to upgrade.