You didn't specify M520 MTB pedal or A520 sport road pedal. The A520 has a sport level cage/frame around it to support the foot more. It is also single sided like like other road pedals.
The mechanism is the same for both pedals so the same shoe and cleat are used. With the external cage, you can occasionally unclip for a minute or so if you want to move the foot around. I use the A520 on the MTB as well as the road bike because I'm just to old to remember by reflex which pedals I'm about to clip into so the same all around makes life easier.