Re:Best belt for deadlifting
All things being equal, I'd try to stay with the thicker belt, unless it is causing too much discomfort by digging into bones. It's good to have the full width at the front if possible. Esp. when bodyfat is low, I have found new 13mm belts pretty painful (which is no good if it interferes with form), but for training, wearing a TK waist band underneath is a great combination - extra warmth during warm-up, and then some useful padding when the real belt goes on.
Senex decrepitissimus, sponsus amicissimus, pater devotissimus.