Don't be shy of the bargin 520's. They work really well, much better than Shimano's past entry-level 515 SPD. The weight penalty over the 540's is in the neighborhood of 60g for the pair. Their finish leaves a little to be desired, as the chrome treatment on mine is flaking a bit after a year of use, and the springs have a light surface oxidation. But all-in-all, they continue to work well and should last me a good long time.
I've got kind of messed up knees, and as wraith indicated, you ought to pay attention to float range and release points during setup. SPD float and release isn't technically adjustable, but what I do is carefully angle my cleats to point in (towards my big toe) just a few degrees to set release where I want it, so I don't have to crank my heel too far out to unclip. Float is generous enough that I rarely experience knee discomfort, but some riders appreciate the greater float that some other brands of pedals offer.
My suggestion is that for learning clipless, ease of entry is the most important feature in a pedal, and price is important too since you're probably not fully committed to sticking with clipless at this point. The old 515's were cheap enough, but I felt they could be a little stickey at times, making them a second tier pedal for a beginner. The 520's I really like in this regard, because they have the price advantage, along with the ease and precision of entry of Shimano's higher end pedals.
. Thanks for the help!
