I would go with AAdvantage:
1. Just like with DL Silver, with AAdvantage Gold (lowest elite tier), you can select extra-leg-room seats for free at 24 hours before departure. (Check-in opens at 24 hours before departure.)
2. The ability to earn AAdvantage Loyalty Points from flying on AS and B6 will be helpful.
3. You can spend your way to status with AAdvantage: Loyalty Points earned through hotel stays, car rentals, credit-card charges, AAdvantage Dining, AAdvantage e-shopping portal, Simply Miles, etc., all provide opportunities to earn Loyalty Points.