in terms of effectiveness I would say. it is probably OK the 10mm (I don't know how much you lift) but as I said there is considerable difference. And also something very strange during DL the 10mm was more painful against the body than the 13mm, don't ask me why
also I was surprised for the difference in size. I am a 33 Levi's size (90cm waist give or take) I wear the large with the screws roughly in the middle, and nearly on the end of the belt on the medium. so as recommended somewhere else, if in doubt seriously go for the larger size, I think it won't fit anymore if I gain some weight around the waist..
hope this helps ask for anything will try to answer considered I have the 2 for now..