Ero, if there's any chance you will be flying 25,000+ miles in one calendar year, I would highly advise the AA card. DL changed their elite program rules last year to make it harder for people such as yourself (i.e. people who fly on heavily discounted tickets) to achieve status. You can achieve Gold status by flying 25,000 miles on AA in one calendar year, whereas you would need to fly something like 50,000 (discounted coach) miles on Delta to achieve Silver Medallion status.

As an AA Gold member, you receive an automatic 25% mileage bonus for all paid miles flown, in addition to other benefits, such as first-class check-in, exit row seats, & upgrades.