In reply to spearing05:
MTB pedals have a smaller cleat that is less likely to get filled with mud, and leaves most of the shoe unaffected for walking.
Road clipless have a big, stable platform pedal (by comparison), a huge cleat and the shoes are typically stiffer, allowing more efficient power transfer. However, they are a complete bitch to walk in and are normally more expensive for both shoes and pedals.
For commuting, you'll probably be happier with MTB pedals and shoes, particularly as you intend to use them on the MTB as well. Road clipless for commuting can be quite a bad idea, as stopping at lights etc. can cause a lot of cleat wear as you clip in/out and put feet down at lights.