Buy the M520, unless you spend lots of time riding in torrential rain or knee-deep mud. Or buy the one that matches the color of your bike best. The difference in price and quality is minimal... I have M520s on most of my bikes, though I did spring for XTR-level M980's on my Cervelo RS road bike. Without a gram scale handy, I couldn't tell the difference between the two models...
Edit: You might also want to consider the M530, which is like the M520 but with a slightly larger platform.
2009 Specialized Tarmac (roadie) and 2011 56cm 600 LHT (touring)
Originally Posted by maidenvoyage
I'm going to REI to buy my first pair of clipless pedals. Do I get the M520 or M540 pedals? Please decide for me...
I'd go with the M520s. The M530s are a bit of a hybrid. FWIW, I have the M780s (very similar to the M520s) and love them for touring. The Shimano cleats let you wear shoes that ride and walk well (I only take my cycling shoes on tours), they are easy to clip-in and out, and they have a little side-to-side wiggle that is easy on your knees.
Do know that learning to ride with clipless pedals is, well, a learning experience. It usually goes okay for the first week or so. And then you'll pull up to a stop light not thinking about your pedals (because you're an "expert" at this point) and suddenly realize that your foot is stuck to your pedal. Rather than swinging your heel outward (which is how you normally unclip), you'll frantically pull up on your foot and remain clipped in. After slowly tipping over in front of all those cagers (i.e., autos) your face will turn as bright red as the tiny road-rash on your arm. It'll only happy a couple of times during the first year, but if you're like most of us, it'll happen. The great news is that clipless pedals feel wonderful. Your legs focus on rotating without any micro-muscle-maneuvers to stay on the pedal, you'll get better at spinning rather than mashing your pedals, and your legs will be happier at the end of the day.
My first pair of clipless pedals were M520s. I loosened the tightness bolt all the way to get used to them and just went up and down my block a few times to clip in and out. The pedal is fantastic and I have been using it to this day - 2500 miles later. I will say if you are doing heavy mountain bike cycling to give them a look over afterwards, as mud, clay, grime can build up. It's a great pedal at a cheap price.