Road shoes are stiffer in the sole and permit the use of road pedals and cleats, which give you a very wide and secure contact platform relative to MTB shoes. If you're poodling, touring or just out for a spin, MTB shoes will suit you fine -- and you'll be able to walk around without looking like a spastic duck. If you plan to go for long fast rides without a lot of stops, road shoes will give you a better pedalling platform, resulting in greater pedalling efficiency.

The reason why the two pedal/shoe systems exist is that they offer different advantages and disadvantages. There is always a tradeoff. I've done a century in MTB shoes, and it took my feet a week to recover. On the other hand, my GF and I had a brief debate over which of us was going to actually go into the grocery store on the way back from our klate-day ride because walking in road shoes is a pain.

Ideally, you'd have bot and a pedal wrench. Going siteseeing on the road bike? Take your MTB shoes and install your SPDs. Going for a 120 km group ride? Take your road shoes.