After a few years of clipless I tried going back to toe straps, and promptly screwed up a knee. My foot would just wiggle out of a good location into a bad one. I like platforms but at the moment, they don't agree with me.
I run with the MTB SPD. Honestly, I'm not that fast, so being able to walk off the bike is a bit more desirable. I have noticed hot spots on my feet, but I've yet to try out different shoes to see if that would fix the issue (or not). Takes more than 20 miles to get a hot spot, and since I don't often have time for long rides, it is what it is.
I run the M520's. I have the M530's on my "fast" bike but honestly I don't think the extra body provided any more foot support--and again, I'm not that fast, so any aerodynamic advantage just doesn't exist. The '520's seem like a good deal. Clip in from either side. FWIW, you don't have to clip in to pedal--taking off from a light I often will make several rpm's before getting clicked in.
My biggest problem is finding a decent pair of shoes. Shoes tend to run narrow, but not narrow enough for me (hence the attempt to go back to sneakers).