Don
Do you mean A530s? M520s can be adapted to be used with flats but I doubt it would last. A520s are single sided SPD pedals. A530s have a big flat on the other side. M324s are also combo pedals if memory serves.
Correct on both counts:
A530
http://www.amazon.co.uk/Shimano-PD-...2?ie=UTF8&qid=1457966506&sr=8-2&keywords=a530
M324
http://www.amazon.co.uk/Shimano-PD-..._UL160_SR160,160_&refRID=1KNZKNVSW448FS3DTSB9
M324 look to be better if you use normal trainers more than occasionally, as the cage looks to have a bit more bite, rather than the smooth A530's.