I've got Shimano PD-M540s on several bikes and they are great pedals. I got the 540s rather than 520s because the price wasn't that much more online if you shop around. The 520s might be every bit as good, but in my experience the more expensive (but not most expensive) Shimano parts have better bearings and last longer.

I've stuck with SPDs over the years because my original set of Wellgo SPDs are at least 10 years old with 10s of thousands of miles and still work great. They are great for commuting because they are double-sided and easy to click in. Tension is also easy to adjust.