United would be a far, far better choice for your circumstances.

As a Premier (2P) on United, you would receive Delta-like unlimited upgrades domestically. There is currently a 49% overall upgrade rate reported for 2P.

On American, you need "stickers" to upgrade - each sticker covers 500 miles of travel. Every 10k miles you fly, you earn 4 stickers. Otherwie, you have to buy upgrades at the rate of $30 per 500 miles of travel. Anecdotally, AA Gold upgrades clear somewhat more often - but if you'd rather not pay, that's no good for you.

Assuming you won't be clearing an upgrade more than 50% of the time, then, United gives you access to Economy Plus. On AA, you'll be fighting for the few exit/bulkhead seats with all the other Golds and Platinums.

For international and partner travel, the benefits are basically the same (few) in either program.

It would be worth your time to look up the specific benefits for Premier and Gold, and also oneworld Ruby and Star Alliance Silver.