The destinations switch fleets on a month to month basis. Some destinations (EGE) are only done on the 319, but they’re few.
Recently there have been a lot of 320 drops. Before that there were mostly 737 drops to MIA. You’d think AA has some competent informed master plan for why that swings the way it does but if you can figure it out, let us know.
If you want MIA, take whatever will get you there the soonest and don’t overthink it.