I've been DL PM (now DM) for a while. 2 years ago I took the AA challenge to Plat, and flew them for a while with that status.

I came back to DL.

Why? The FF benefits aren't the only thing that count for me, although they are important. The personnel and in-flight experience also count for a lot, and in this regard I feel DL's people are the best. The route map and everything else everyone has been discussing in these threads matters, too, but it is different for each person.

I don't have a company paying for my tickets, it's just me and my credit card, and DL continues to earn my money.