Like Qwkynuf said, look at where you are and where you want to go. If you're in / near Atlanta, Detroit, Minneapolis, NYC, Seattle, Salt Lake, or Los Angeles or a few others, Delta is a good option. Find out which airline best services your locale. If you plan to move for a job, etc, know where you're going and do likewise.

If you're not sure, get the CSR.

I have the CSR and Delta Reserve and Platinum cards (four of them, in fact). I'm a Delta loyalist, but only as long as they continue to treat me well. Being based in the HSV / BHM area, I could choose anyone. I just happen to be hugely invested in Delta at the moment, so a divorce is likely only in the event I become a battered spouse.