More advice
Now that I've replied to the wrong question, I'll point you towards another thread in this forum that has to do with reliability. There is a thread which has a poll on the reliability of hydraulic vs mechanical disc brakes. Read this along with the FAQ section on discs. Again, the choice becomes one of preference, with advocates on both sides being very passionate about their choice. I only have experience with mechanical discs on bikes, and so far, that experience has been very good. If you go this route, your best choice by far, and this is a consensus opinion, not just mine, is the Avid BB7, hands down. If you go hydraulic, there appears to be a great disparity regarding peoples experiences and preferences. Just be sure that you choose a hydraulic brake that has a better than average reputation.
Again, best wishes in your selection. Make sure it's money well spent.
Bob