I doubt you'd notice much difference in pedaling efficiency between the two, unless you race or use a power meter.

I agree that you'd notice a huge difference in walking comfort.

I have two pairs of SPD shoes:
Pearl Izumi X-Alp Seek V (summer shoes; flexible, comfortable, very easy to walk in)
I can wear the Pearl shoes all day at the bike shop, where I'm on my feet. They feel almost like normal sneakers.
SiDi Diablo GTX (winter shoes; stiff, much tougher to walk in)
I hate walking in the SiDi boots; they're super stiff and the sole is hard with little traction on smooth surfaces.


Regarding the pedals you mentioned:
The A600 (like the A520) is a single-sided SPD, without a platform surface on the other side. That seems like a pointless hassle, unless you insist upon saving the weight compared to the 2-sided M520.
The A530 has a platform side and an SPD side, so you could ride in non-cycling shoes on occasion. Note: the platform side of the A530 isn't very grippy; my sneakers would slide around when wet. I drilled and added spikes (grub screws) to my A530s.