I run mountain bike pedals on my tandem. The stoker finds the M505 pedals easier to clip out of than our A520's and as a captain I really want a double-sided pedal, but I suspect these were bottom-of-the-line when available. Other than the weight advantage can someone comment on the pros/cons of the various double-sided SPDs (M540, M520, M770, M970)? The 540 and 770 show as 350g/pair, the 970 as 325g/pair. Shimano doesn't list a weight for the 520 which are no doubt heavier. Are there durability differences? Do the more expensive variants have smoother mechanics?