Float is the ability to pivot your foot towards the left or right. It compensates for the inability of your knees to track in a perfect vertical plane. SPDs have 4 degrees of float, Looks have 9, Speedplays have 15 or more depending on the model. Depending on the condition of your knees, you may need more float. I have Look pedals on my road bike and SPDs on my MTB and on the Spinning bikes I teach on. I have never noticed a difference in comfort, but I do prefer the larger platform of the Look on long rides.