We've flown both those routes many times. We slightly prefer Delta, but never had major problems with USAir (AA) to SXM other than slight delays. Delta has a better on-time record and we find the service a bit better. However, with that being said, if I had a choice of first-class seats over economy at the same price, it would be a no brainer.

Edited to add: I don't really like the smaller planes either, though they fly fine. I'm just a bit claustrophobic and prefer the larger planes.