Seems like every week someone asks the "road vs mountain shoes" question, so I got tired of repeating myself and wrote up a whole thread about it. :-)
If you want all the detail I could possibly think of from years on the forums and using both myself, read this - "Choosing 2 hole (mountain) shoes vs 3 hole (road) shoes"
http://www.bikeforums.net/showthread...oes&p=10936693