I have tried both SPDs and platforms on my mtb and I've gone the way of platforms. I had Speedplay Frogs on my mtb but I found the pedal is NOT good at shedding mud which can make clipping in a little difficult. On the upside they were the easiest to clip in and out of. If you decide to go with platforms, the 5.10 shoes that Irulan posted have a super sticky sole and will really grip the pedal. Just know that you will have to deal with the pins kissing your shins and you will want to plan accordingly.