In my opinion, hydraulic brakes are overkill for a dirt jumper, unless you specifically need them for the terrain you ride (for example, Downhill).
Avid BB7 mechanical are plenty strong. Mechanical disc are already stronger than BMX brakes ("u-brakes"), and many bmx'ers don't even ride any brakes at all.
if youre on a budget, go with mechanical. i run a hayes mx2 with a linear cable and it locks no problem. cheaper to maintain too, if you bust a cable its like 15 bucks and a lot less hassle. i agree that bb7s are probably the cream of the crop for mechanical