Are fixed or floating SPD-SL cleats most suitable for a long commute?
Fixed position cleats, or 0 degree float cleats, require far greater precision about cleat setup on the shoe. Failure to get the setup right will mean pain, and can mean injury. 

That is also true of floating cleats. Most pedals come with cleats that have between 4.5 and 9 degrees of float built in. I don't know of any pedal which has a 0 degree cleat, stock. You would have to buy it separately. 

For your purpose, there is no benefit to a fixed position cleat. They are primarily used for track racing and for sprinting specialists in racing. 

Given that, floating cleats are most suitable for a commuter's bike.