Commuting - Clipless Shoes...MTB vs. Road?

I have decieded to try clipless pedals. What are the differences between the MTB vs. Road clipless shoes? Other than the knobby soles, are there any differences on riding performance. I have been looking at various shoes and although I ride the roads, some of the MTB shoes look appealing.

I prefer to use one set of shoes and pedals for all my riding so I put time atac xs on the road bike. The difference, with sidi mtb shoes, is more sole flex. However, if you go for the high end srs shoes then not only are the soles replaceable but the soles are much stiffer too. It's probably not a perfectly efficient combination but it's very comfortable and does mean I don't have two or three different pedals and shoe combinations.

If you'll be commuting with them and then walking into work with them, go mtn.
Road cleats make you walk like a duck and are noisy -- clip-clop down the hall.
Mtn cleats are slightly recessed and don't affect your walking and don't make noise.
Both work well for riding.

My choice for weekend rides is Look pedals and cleats on road shoes.
My commuter has Egg Beater pedals and I can walk all day in my mtn style shoes (http://www.exustar.com/sm600.htm) with cleats and no one knows I'm wearing cycling shoes.