I certainly prefer AA to DL with regards to earning FF miles. I fly about 90% using FF miles, or Jumbos of one sort or another. So I go wherever it makes the most sense comfort-wise and dollar-wise.
As I've said for years, the airline business is cyclical. I stuck with NW through thick and thin, until the merger was announced. It quickly became obvious that someone else was driving the bus after that, so I booked everything I could using miles while they still had real value.
I didn't buy a DL revenue ticket for years. I finally got to the point that I'd buy tickets if they were the cheapest and most convenient.
One day DL will be hurting for customers. I have a long memory, though, and doubt that I'd ever be loyal to DL. Of course, I expect AA to downgrade their program too. It's the nature of the beast these days. I hope they aren't as weaselly as DL.