The A320 family being newer is significanlywider than the 73/75 series.And by significantly I mean a wider seat that you notice and a wider aisle too.
I've recently flown on A320s and 737-700s. Frankly, I didn't notice any difference in the width of the aisle or seats. I'm not saying there isn't a difference, just that it was small enough that I did not notice it. YMMV.