Our pricing was actually incorrect. Someone (Totally not me...probably me) got slap-happy with their keyboard.
The only real difference is that Magnaflow Part# 15591 is for 2011-2012 Mustang 3.7Ls. Magnaflow Part# 15153 fits 2013-2014 Mustang 3.7Ls. Magnaflow are real sticklers when it comes to the look of their kits, so they designed a version for the 2013+ Mustangs for a "proper" tip placement with that car's lower valance.