From everything I've read, I think the answer is that AA isn't planning to get any A320s in the initial batch of 130, just A319s and A321s. 738s are directly comparable to A320s in size/capacity but with better range and significantly lighter weight than A320s. With lots and lots of 738s already in the fleet, the A320 would add no advantages over the 738 (other than the wider fuselage and higher weight, both of which come at the price of fuel economy).

A321s are closer in size (floorplan) to 757s than are 739ERs and are significantly lighter than 757s (but heavier than 739ERs), so I see the tradeoffs and the thought process about the A321s. AA wanted a plane that more closely matched the 757 in size than the 739ER. Delta and United recently placed large orders for 739ERs and not for A321s, but AA likes to be an innovator.

The part I don't understand is why the A319 instead of 73Gs? The A319 is heavier than the 73G, so AA is signing up for a long-term fuel economy penalty right out of the box. Those six inches of fuselage width and heavier weight than the 73G come at the price of fuel economy. Maybe purchase price and availability played a part.