Float or no float...

What my idiotic swimming teacher said in primary school as he threw me into the deep end....

Seriously, it is personal choice and I think it is independent of cadence. I ride Speedplay X1's (lots of float) and I have heard that they are not as efficient as float limited or non-float pedals. However, I have knee problems and riding Shimano SPD (some float) aggravated them. I'll trade any power lost to the Speedplays for the benefit of happy knees.

Like a lot of people said, if you go no float be absolutely sure to have a thorough cleat fitting done by a bike-shop. For systems that have lots of float a cleat fitting while desirable is not that important (on Speedplay X series, since there is limitless floats your feet just adjust to their natural angle).