In reply to Removed User:
SPD
Pro - lighter, possibly need fewer shoes, should you walk in mud, easier to clean out as you clip in, , 'easier to walk in' (not sure I think there is enough difference personally)
Neg - Not as comfortable over long time/distance, less power on the stroke, more lateral movement (knee problems?)
I have SPD on the mtb and the cx but SL on the road.
I wouldn't put spd pedals on my road bike, I stay clipped in for longer periods and so the comfort and power matters.
Why not consider a pair of MTB shoes and a pair of road shoes.
RE: TomB, which brand of cleats, because going through a set in a month would be odd. I Probably go through my Shimano yellow once a year, riding 4 times a week walking to and from places in them, essentially not taking any care