My road pedals are on their way out and I am not keen on the whole road pedal thing anyway so I was thinking of putting some SPD530s on the road bike, the ones with a sort of platform. My road shoes will take 2 bolt fixings as well as three so the cleats will fit o.k. Surely most of the stiffness comes from the shoe anyway so what will I lose by going to SPDs? Apart from rapping my nads on the cross bar when the Look cleat skates off the pedal of course.

http://singletrackworld.com/forum/topic/shimano-pedals-and-dhb-shoes-clearout-road-and-mtb

I went from CB Candys to Shimano 105 pedals and the latter are far more secure feeling, and provide a much better platform IMO.

I can’t comment on the SPD pedals you mention, but I won’t be going back to mtb pedals on the road bike.

Mtb shoes prevent the cleats from making contact with the ground, so they are easier to walk in. Road shoes with ANY cleats are not as comfortable to walk in as they stand proud of the shoe and you are walking on the cleat, which means they wear out quicker.
other than that there is no difference