I have both of these pedals (M-520's and A-530's) and have used them with my Shimano M-162 shoes.
I was in your position about 18 months ago...
I started with the A-530's (same theory as you), as these were my first clipless pedals and I wanted to have the flat-pedal option for when riding in the Melb CBD (as I was nervous to be able to unclip in a hurry).
I used these for about 6 months or so and they were perfectly fine. You could ride on the flat side with the MTB shoes but they were a little slippery, but fine for short periods of time.
I found the way these pedals were weighted I could pedal forwards with one foot and the clip side on the other pedal would point upwards at the top of the stroke so I could clip in without looking down - not sure if I was just lucky or whether this is a design feature but they worked fine for me and I rarely had an issue finding which side to clip into.
Perhaps they may be slightly chunkier than other platform/clip-in pedals but at no time did they cause me any issues.
Then I decided that clipping in and out was no longer an issue for me and wanted something a bit less "newbie-looking", for a road bike, so I went with the M-520's.
The M-520-'s really don't feel any different to the A-530's once clipped in and riding and they seem to do the job just fine. They certainly look much better on a road bike (compared to the A-530's).
I had been tossing up recently whether or not to move to the SPD-SL's (as occassionally on long rides I sometimes get hotspots and sore feet), and co-incidentally I came across a bargain on the weekend that I couldn't pass on - Sidi Genius 6.6 Carbon Lite Vernice for $259, normally $420, so I have these on order along with some 105 SPD-SL pedals.
I would say go the A-530's and use these until you feel you have no issues whatsoever in clipping in/out and then down the track you can upgrade to the M-520's.
The cost of these pedals (in comparison to what you will end up spending on your cycling in general) is pretty insignificant and it gives you something to lust after and upgrade - which we are ALL guilty of
Hope this helps...
Dave