I use the bottom bracket type magnet on two bikes and though I only use the Quarq on my TT bike (use Powertap on the other) the magnet on the bracket has been solid on all types of terrain and environmental conditions.
its really hard to keep them fixed due to the vibrations through the frame and the impacts from potholes etc. The bottom bracket ring mounted magnet is ideal if it fits but it wouldnt fit one of my bikes. I have a neighbour who is a flooring fitter and he gave me some flexible super strong flooring adhesive that did the job brilliantly. My frame was a matt finish that made usual adhesives extremely difficult to use.