To my mind there is absolutely no contest between mechanical and hydraulic disc brakes. Hydraulic is definitely the way to go. Virtually maintenance free, with no cables to stretch or get corroded and gummed up. The mechanical discs I have encountered never seem to have the power of a hydraulic system either. Had Hayes hydraulic discs (with DOT 4) on my Scott MTB for nine years now and the only thing I've ever had to do is change the pads a few times. Still got the original brake fluid in the system and everything still works fine. I use medium compound
EBC pads, which give a good balance of stopping power and longevity. I occasionally take the wheels off and give the rotors a good clean with a recognized disc brake cleaner, but that's about it for maintenance.
I'd have to say that the Shimano hydraulics on my son's MTB have fantastic stopping power (more so than my Hayes brakes) but, in my opinion, not quite such good modulation.