So far I've used SPD and SPD-SL. I greatly prefer the SPD-SL system for road riding. However I woudln't use it for moutain biking as it's not that easy to walk in the shoes. The main reasons I prefer SPD-SL are that it's easier to clip in and out, it's larger platform seems more comfortable to me but that also might be because my road shoes cost 3 times what my others cost and are stiffer, and my feet are closer to the spindle and I feel more one with the bike.