M324 pedals come with the SH-56 cleats. I've only ever used the SH-51 version and, as I need to pull up quite hard on hills (fixie, can't change down) I don't think I could rely on the 56 cleats.
The 51 cleats have never failed to release on demand, and they tend to let go in a crash too, which is more than I can say for some road pedals. I've been using them for 25 years and they're pretty much perfect.