float allows your foot to twist side to side a bit without popping you cleat out. this is good if your leg doesn't move in a perfectly straight motion. for example, if when you pull up your heel goes out and when you push your heel goes back in, then float is good because it allows for this. if you have no float, your knee will compensate for your foots desire to pivot.

now if you have awesomely straight knees or you had an awesome bike fit, then no float is nice because it won't let your feet twist you out of your fit.

it's really a preference in the end. do you want your feet pretty much stationary or do you want them to have a little give from side to side.