Cleats
SPD - Metal cleat uses 2 mounting bolts - Used for Mountain bikes
SPD-SL - Plastic cleat uses 3 mounting bolts - Used for Road bikes.

SPD-SL mount on the bottom of the shoe. The shoe itself has none/little in the way of ridges/grip. Because the cleats are not recessed into the sole it makes it awkward to walk.
SPD are recessed within the sole of the shoe. this leaves the ridges/grip available to walk on. Better to walk on, but the soles do tend to be on the stiff side. So don't toss your hiking boots just yet.

IMO if you plan on walking much, SPD is the better way to go. My fixed has SPD's due to my using it for commute. I have to walk the bike within the Train stations.

For the road bike SPD-SL all the way. Although I do carry Cleat Covers with me. These make SPD-SL even worse to walk in, but you don't tear up the plastic cleats if you have to walk on them.

Hope that helps

Barry