If you haven't already you should read through a number of the threads in this forum.

DL planes & FA's are comparable or better than AA IME. However, with DL comes delta.com where you will find it difficult to book revenue tix and even more difficult to book award tix. A subscription to EF and time to work through everything yourself is important. Elite phone CS is inconsistent at best though seems to have improved recently. DL makes far more schedule changes than other carriers, makes them closer to flight time, and does not preserve reserved seating in many cases. IOW, you have to be careful to what extent you rely on DL getting you somewhere, especially if it involves a connection.