Double or single sided?
It makes no real difference, even if you ride clipless all the time, it's not hard to flip a pedal.
As far as using the platform side, that depends on the amount you wear your shoes off the bike.
SPD shoes can be walked in and are quite comfortable. However, the sole does wear down and if you do a lot of walking, you can wind up walking on the cleats which isn't a lot of fun. Added to that, stiff soled shoes aren't real good for sitting around inside. Cycling shoes are great on the bike, off the bike, even SPDs get a tad annoying.
There are solutions to mixing riding and a lot of walking.
The most obvious is to carry a pair of walking shoes and change at the end of the ride. This is fine if you don't mind carrying stuff and mucking about with your clothing.
The second is to have a pair of walking shoes at your destination. That's fine if you have that luxury, a lot of us don't.
The third is not to wear shoes at the other end. Where I've been doing volunteer work, one of the bike commuters gets around the office in his socks all day. Weird but true.
My solution has been to go to the new style mtb or bmx shoe. They have a flat sole, some stiffening for support though not a lot and can be lived in all day. They suit me because I wear one pair of shoes that allows me to ride but can be walking in all day over some quite long distances (5km on pavement in SPD shoes soon shoes up their limitations).
As far as pedals goes. I don't like plain platforms. I've got the M324s on two of my bike but tend not to use the platform side because I don't like the floaty feel of platforms and prefer to take the time to change into my cycling shoes for short ride up to the shops. I've tried the mtb pedals with all the little studs but don't really like them either. My commuter now wears toe clips.
Interestingly, my fixed gear bike currently has M324s and I don't mind using the platforms on that (as opposed to freewheel bikes where I hate them) but will probably go to toe clips pretty soon. My sports bike currently has M324s but is only ridden with clipless shoes - she may change because I want to use her as a commuter now and then.
The clipless system is great for positioning your feet, but that is only a good thing if your cleats are in the right place and considering they are still discovering where the 'right' place is, I'm not sure that's as great an advantage as people imagine. Certainly, getting the cleats wrong can cause quite a few problems and no, the ball of your foot over the pedal spindle is NOT the right place - like KOPS, it's been disproved.
The clipless system is great for keeping your feet in contact with the pedals but does it really lead to more efficient pedalling? The claim is 'yes', but how much more efficient is up for argument. A lot of people imagine that they gain power by pulling up on the pedals but that has been disproven by strain gauge measurements - unless you are an elite athlete in a full on sprint, the most you are doing is getting your foot out of the way of the rising pedal.
Clipless does add a huge amount of confidence to your cycling ... especially when turning insane revs on your fixed gear bike down that long hill.
In short though, you don't need clipless and it's not doing a lot for your cycling, despite the claims made by marketers and the race boy set. However, the intangible advantages of being secured to the pedals mean that if all you're doing is riding the bike, clipless is a very good way to go. The other advantage is wearing shoes specifically designed to support your feet and I believe this is a bigger advantage than the presence of clips be they toe clips or under your feet. Interestingly, the concept that stiff soled shoes are the best is also under fire now.
If you want to do some walking, SPDs or Crank Bros are a good way to go.
If you want to do a lot of walking and want/need to do it in your cycling shoes, it's toe clips or platforms - SPDs will do it for a while but there will come a time where they'll start to annoy you.
Now that I've got good cycling shos with flat soles (mine are Shimano bmx shoes as it happens), toe clips have proven to be the best option for me. But it really does come down to personal likes and lifestyles.
Richard