The m520 with clips on both sides are my pedal of choice. As I commute si much, being able to clip in on either side is a real advantage, but can see how the one sided ones have a certain flexibility for foot wear, although like for staeph, personally it's not a big issue and short rides are OK for me even with sandals.
I think the main positive with shimano pedals like all of these is how well the bearings stand up over time. I've mentioned before but my old pair of two sided ones I bought in 92 are still being used and have very little play in the bearings. Very impressive lifespan.