Given your size, I would probably recommend the 10mm. It should have all the support you need. The 13mm seems to be more popular with bigger guys who are squatting huge weights. The 10mm is be easier to break in and won't dig into your ribs or hips as much.
Also, get a single prong and not a double. If you really crank the belt down, the 2 prong is a nightmare to get off.