I did go ahead and book through Atlanta on Delta. I did book the Delta preferred seating which gets a seat closer to the front of the plane, right behind first class. That was to get off the plane quicker. It was still cheaper than American through Miami.
I did talk to Delta and they thought 45 minutes would be ok. They did tell me that typically the SAT flight arrives in terminal D and the SXM flight leaves from terminal B, not the international terminal.