I was in the same dilemma: I had to choose between Shimano PD-M520 and Shimano PD-M540.

In order to decide, I looked at the diagrams provided by Shimano:
Shimano PD-M520 has a slightly different design than Shimano PD-M540, which shares the same design with the more expensive Shimano PD-M770 (Deore XT) and almost the same design as Shimano PD-M970 (XTR) - only the "body cover" has a slightly different design on XTR pedals. The difference between those 3 models (PD-M540, PD-M770 and PD-M970) is given by different materials on some parts and/or different finish on some parts.

Also, PD-M540 were only slightly more expensive than PD-M520 at the shop where I purchased them.