I left DL for AA just last week (many of the same reasons in this thread), however I have to say in fairness the Delta.com and DL TechOps is far better than AA in many ways. Just to name one, miles post in your account before you even land at your destination.

Are you buying regular economy fares or are you paying high or full fare?

After reading your question I was thinking that you probably are buying regular fares L,U,T and understand that you will only earn 1/2 MQM on these fares, which means that you will only make the lower elite tier on Delta. This will give you far fewer upgrade chances, and you will earn less FF miles as well.

My understanding is Delta's changed its program this year and is really only good for high fare customers. Delta is arguably one of the worst for regular paying fare customers who fly regularly.

If you fall into this category, AA should be miles ahead.

I actually pay high or full fare most of the time. In fact, with a Y fare, I get either 1.25x or 1.5x credit times a 750 mile min per segment, which is a pretty good deal.

Got it,
That was a pretty big point. Believe it or not, probably you are in the minority of most..... especially here on FT.

Probably the opinions of many of the people on this board, buy either cheaper fares if they have a choice, or purchase a mix of business when appropriate or low fares at other times, which would explain the strong sway towards AA. Delta's program probably competes fairly well for the few high or full fare flyers. I wish I worked for a company that didn't care how much of their money I spent for travel.

Have you also looked into some of the F fares or discount F fares. That may also change your opinion. Finally, it is possible that being loyal may really not make too much difference in your case.

All Delta coach fares earn 100% regular FF miles. Only MQMs are at 50% for LUT fare classes.

Where do you stand in overall program miles?

AA will be easier to qualify for MM, since all miles from any source count. DL isn't even "butt-in-seat" miles anymore; they only count MQMs now!

AAs food is definitely better, too

If I were forced to fly either of the two, my pick would be American. I've had some bad customer service experiences with Delta and pretty much shut them down from consideration on future flights.