Once you hit 25K a few days later status kicks in and you get the benefits.

If you are planning on LH flights it's a no brainer to go with United -but AA would give you BA and Iberia . Really depends on where you are going to be flying next year.

I've had gold and plat with AA in the past (worth checking out their challenges-that might work best for you) and am currently silver with CO (which now gives me UA too as you can link UA and CO FF accounts). For a mix of US and Europe flying Delta had a lot to offer -unless you want to redeem miles -their redemption is horrible.