I use mountain SPD pedals on all of my bikes. Some with Shimano, some with Eggbeaters (need different cleats so I have two pairs of shoes). I'll convert all over to eggbeaters at some point.

The SPD road setup uses different cleats and different shoes. The cleats are bigger to try to prevent hot spots. But even my highly walkable touring shoes I use never give my feet any problems. And all the road shoes - like the "race" MTB shoes - are completely rigid so I don't see where it makes any difference.

If you're just trying them for the first time I would suggest MTB SPD anyhow - you'll probably like the selection of shoes better.

Interestingly I have Shimano road pedals that actually use the MTB cleats. Not sure if they still make those or not. Handy though for not having a different pair of shoes for each bike.

C