I've kind of eliminated United from the list, which was my previous top choice.
I am leaning towards Delta now. The only problem is routing. I live in Tampa. There are almost no direct Delta flights to the destinations I am interested in visiting soon that do not involve a stop in ATL. For example, I usually fly into BWI when visiting up there, but neither Delta or American have direct flights there. Delta has a stop in ATL and American barely flies there at all.
Also, which has a more updated fleet? I know AA is going through the whole fleet revival, but from what I can tell on my routes, I'm going to be stuck on lots of MD 80s and 757s with no personal TVs. I know most of the DL 757s came from Song and have TVs.
Finally, why is Delta so much more expensive in some markets? AA flying up to CLT then to SJU is half the price of Delta flying to ATL then SJU.
For the FF programs, the easy earning of AA is attractive while the no expiration of DLs points seems nice.