Hopedalehills has the right idea. The CrankBros eggbeaters are excellent on and offroad pedals. They are simple to set up and work flawlessly. Get one good pair of shoes and use them for both bikes. I chose Sidi Dragons because (1) they fit my feet perfectly and (2) the soles are replacable, so when they eventually do wear down you just order new parts and replace them yourself.
I used Speedplays on my road bike for years and they're a great, lightweight pedal, but you can't overestimate the practicality of being able to walk normally (and safely). I went from Sidi road shoes and Speedplays to Sidis and eggbeaters and could detect absolutely no difference in performance -- even while racing, except that I can walk to the post-race food table with ease ...
Highly recommended combo.