The 6 inches on the 320 vs the 737 do offer significantly higher stiffness for weight, noticeable space / comfort.
I think one of the biggest advantages of the A321 over the 737-900ER is payload range and the ability to carry cargo containers and pallets, while the 737 and 757 can not.
Because the greenliner is just over 60 meters, a lot of them should fit in.
The A321 can't match the 757 range flexibility it is build for / used on high volume trunk routes. Its cargo capability, much lower weight and operating costs was one of the factors that pushed the 757 out of the market.