I would always wear a belt for work above 85% your abs will get stronger until you get to that point.
I mean, who's got stronger abs the guy that squats 800 with a belt or the guy that squats 400 without one?
I like buckle, single prong.
Any of the major powerlifting companies make good belts, there's not a whole lot of difference between them, to the best of my knowledge, just how think you want it.
I like mine >10mm thick.