I use SPD's out of convenience on my tandem as well as my road/cyclocross bike but would suggest Look pedals if you have knee/foot comfort concerns. The huge platform connecting you to the pedal spreads the pressure across your foot and the swivel alleviates alignment problems with your knees. They don't make noise like speedplay (I am not a speedplay fan) and you can actually pedal with street shoes on compared to eggbeater. They aren't great to walk in as are the Shimano SPD's but they have plenty of pedals to choose from. The negative for Shimano SPD is the tiny interface you have with the pedal that causes a hot spot on your foot. Doesn't bother me but I could see it being a problem.