Hydraulics are IMHO overkill for anything but downhill racing. Too complex to maintain and repair. I replaced my Hayes MX1 with Avid BB7 and I like them: they can be adjusted with better precision and are more powerful than the Hayes brakes. Me, my bike and my commuter cargo are around 240lbs and BB7 will lock my wheels if I squeeze too hard.

If you're gonna haul tons of stuff on your BD the hydros may give you an edge but you'd probably want a larger rotors. I don't know what's the max rotor size Big Dummy frame and fork can handle. Since you already have them...

A.