Are fixed or floating SPD-SL cleats most suitable for a long commute? After much procrastination, I've finally jumped in and bought myself a road bike, under the cycle2work scheme we have here in the UK.

I opted for a Boardman Road Race, as it seems to be the most affordable entry level road racer. Since my commute is 20 miles one way (aiming for twice a week to start with) I've been advised to invest in some SPD-SL pedals.

I've never used a road bike, or SPD-SLs for that matter, however from what I can tell there are two types of cleats, floating (which give a little movement sideways) and fixed (no sideways movement).

For such a commute, which type would be most suited? I'm thinking that floating probably for the extra range of movement which could be more comfortable for a longer commute.

Is there anything in it? Or is it just down to user preference?

p.s. Any reason these pedals/cleats would not be compatible with the above bike? Or are pedals universally OK
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.