Speedplay doesn't make track pedals, what model exactly are you looking at?

Speedplays are good for quick on/off the bike, they're very easy to engage but for riding fixed I would use something with adjustable tension. What I never liked about this system is that the release mechanism is located on the cleat itself and you have to lube these quite frequently to make sure they work properly. Never get mud on them or you'll never be able to clip in.

Tried and true Shimano SPDs are awesome, i run them on my fixed and my road bike and have never let me down.