I have used the Shimano Ultegra SPD-SL pedals for a few years, and now use the Look KEO Carbon Cro-Mos (about 6 months so far).
SPD-SL:
+ very stable and grippy to walk in
+ quite easy to clip in and out of by 'foot feel'
. relatively heavy pedal system compared to what's out there now
- the black rubber friction pad is on the pedal, not the cleat. If you don't replace this before it's too late, you ruin the pedal. Of course this wasn't written in the instruction manual (I got these when the Ultegra SPD-SLs first came out). I learnt this the hard way, cracked the ***** and that's why I bought Keos as a replacement.
Look KeO:
+ light!
+ the rubber wear-pad is on the cleat, not the pedal, so you can't kill the pedal that way. It wears along with the rest of the cleat.
+ when clipped in, they feel more firmly attached
. they aren't quite as easy to clip in by 'foot feel' than SPD-SL. still not difficult though
. they look cool
- the cleats are slippery-er to walk on compared to SPD-SL
My recommendation:
At the end of the day, the main difference I've found with these two systems is:
a/ walkability
b/ friction pad wear silliness
If you see yourself having to walk a fair bit, e.g. if you regularly ride to work and have to go through a front door, foyer, up a flight of stairs, etc. before you can change shoes, go for SPD-SL. And be careful that the friction pad doesn't wear too much.
If you're going to be 'only riding', rarely having to really do any walking more than a short distance, go the Look KeO. And be careful not to slip on polished floors
(Don't get the older Look pedals confused with the KeOs when doing your looking about, by the way. The old Look pedals are pieces of junk compared to them, or SPD-SL, for that matter)
BTW: you can pick up any of these pedal systems you mentioned (including the Time Pedals free_rideman suggest) really cheaply from probikekit.com
They'll save you at least 50% off what the local shops sell them for.