You ignored the 720's larger battery, the 635's arguably worse camera, the 635 being slightly larger and thicker, and the software keys on the 635.Here.
Nokia Lumia 635 - Full phone specifications
Nokia Lumia 720 - Full phone specifications
It's not too hard to go over the specs and see for yourself.
The biggest differences that matter as far as I can see.
Bigger SD card in 635, larger screen and faster CPU.