What exact configurations are you comparing? AA international 757 vs. AA A319? What cabin are you currently booked in? Would you be booking outright in J or Y? Would you be hoping for an upgrade on a Y booking?
Lots of variables here. My instinct would be to book the A319 assuming an MCE seat was available. It's wider than any of AA's Y seats. But if you're thinking J - either confirmed or upgraded - and the 757 international config is in play, that might be worth doing.
If everything shakes out somewhat equally and total times are similar, I'd rather fly through DFW than MIA.
I enjoyed Googling A317.