I've used M324's for years, they are absolutely excellent go anywhere, do anything pedals. easy to spin dependent on whether you're cleated or not and they do stay whichever side up when you unclip or stand off at lights etc.
Avoid the MTB style M424 with the spd mech on a rocker inside a plastic cage, they are supposed to be usable either way up with either cleats of plain shoes. The spd mech is supposed to rock out of the way when you're wearing normal footwear but doesn't. They are c**p, you can clearly feel the spd mech poking through, its not a good feeling underfoot.
I've also got M520 straight SPD pedals on my folder with a pair of these
http://www.cheapbikeparts360.com/products/shimano-spd-platform-and-reflector-set/ because I'm quite obsessive about having pedal reflectors, surprisingly grippy.