For those riding downhill, hydros are preferable as they self adjust. The other benefit of Hydros is that most of them have two pads attached to pistons. Most mechanicals (save IRD Dual Banger) have a fixed pad and a moving pad. As it turns out though, a good set of mechanicals is good enough. With the BB-7s you need only turn the nobs occasionally.

On to braking force ... Mechanicals and Hydros have EXACTLY the same braking force when adjusted properly on the same size wheel with the same size rotor with the same pad compound. You may think that the hydraulic brakes in your car are so powerful because of the "hydro". What makes these so powerful is that the brake system uses engine pressure to amplify your braking. On a bike, it's just your hand. You can increase the mechanical advantage, but this reduces the travel of the pads. In the end, the energy put into those brakes comes from your hands. If you want "more powerful" brakes, get bigger rotors.