6 of one half a dozen of another, IMO. But this exact discussion is going on here:

American or Delta?

negril.com/discus/messages/103/142407.html…

We like Delta. We fly out of Grand Rapids at 6:25 AM to Atlanta, have a 1 hr. 9 minute layover and get to MoBay by 1:30 PM. It's the best we can get up here. We've flown Delta for the last fifteen years with no real problems. Their on time status with us has been excellent. They left a bag once in Atlanta on our return flight but delivered it to our doorstep the next morning.

I've used them both for flights to Jamaica. Neither really stands out over the other. We were stranded in Atlanta on the flight coming back due to weather. Instead of cancelling immediately Delta continued re-scheduling the flight until about midnight. This was quite annoying and did not allow us to get a hotel room as by that time they were all booked throughout the ATL area. The weather was not their fault but re-scheduling the flight over and over again was a poor choice by management in my opinion. Other flights with Delta have been alright. We did not have any delays with American either. We've had our best experiences with Spirit but all airlines have their problems.

I have to agree with MichiganNegrillover. We just used Delta for our trip and they were great. Got to MoBay before noon after flying through Atlanta. One of our bags was also delayed on the return trip, but it was also delivered to our doorstep first thing in the morning. In their defense, they got the other 3 bags through a tight connection in Atlanta and the last one was actually the TSA's fault (there was a "random inspection" card in the luggage when it arrived). As far as rescheduling, you'll get that with either one.