the Shimano 324's is their only pedal that comes with a multidirectional release cleat standard, which makes the 324 an excellent transition pedal for riders just starting to use clipless, add versatility for riders who will ride often in street shoes, and may provide an advantage to the less abled riders out there in traffic and start and stop city riding, by providing a solid platform to use without having to get 'clipped in' right off the traffic signals.
The A520 is a 105 level polished, single sided 'road' pedal that uses Shimanos compact mountain cleat. Great for club riders that want off the bike walkability