I just switched from SPDs and mountain shoes to Speedplay Zeros and carbon road shoes. I actually find the Zeros easier to clip into than the SPDs now that they've broken in a little, and they feel much more secure, especially on up hill sprints when I am really pulling up on an upstroke. I ditched the SPDs because I had a couple of unintended releases, one of which almost resulted in my foot going between the spokes of my front wheel.

Comfort wise, on the bike they are similar. Off the bike the mountain shoes are better but the road shoes are good enough.