When deciding what road pedals to purchase, the decision is usually first between Speedplay and Look pedals. After that it's just details (weight vs cost). Having trained and raced with both for many years, I can confidently say I prefer Speedplays. Reason is it has advantages in racing, and specifically at the beginning and end of races.
AT THE START LINE
If you've done it right and gotten yourself one of the front positions at the start line, you have to hold that top 10 position for 3mins or so after the start while the immediate jitters of the other racers get pedaled out. Often there's shouldering between racers who are trying to clip in, hold their line, and pedal - all at the same time. Nothing aggressive, you're just packed in like sardines.
In a road race everything generally settles down pretty quickly, unless someone's too excited and goes off the front immediately (which, if no one sees them as a threat because they're teamless, etc., can actually work). In a crit, the frenzy at the start usually doesn't let up for about 10mins as racers are trying to be in the top 4 to 10 positions going through the corners. I have had trouble clipping in when I was using Look pedals at the start of races, and have quickly found myself from the front to the back of the peloton in 10sec.
That has never happened to me with my Speedplays. Even if I don't get fully clipped in on the first rev of the crankset, the contact is good enough that I can keep pedaling with decent power and not lose position. All you have to do is stomp down into the pedal - no looking, no flipping the pedal into position.
AT THE FINISH LINE
When the peloton is charging towards the line, and suddenly everyone is out of the saddle wrestling with their bikes to get every last ounce of speed out of it, you want to be sure that you're firmly attached to it. Speedplays have always felt like the connection to my shoes is way stronger than they were with Looks. I don't have any proof that that's the case, but it sure felt that way.
LOOK DOES HAVE A FEW PERKS
I actually like the way Look pedals look, and I like their branding. I'm a fan of the Mondrian design. I also like that if you're wearing street shoes they are still pretty usable (Speedplays are no fun with street shoes). Female cyclists I've spoken with have also said that they aren't able to clip into Speedplays so they have to use Looks.
If you're a racer, and especially if you race crits, I can't imagine why you'd opt for Look pedals over Speedplays. For the recreational cyclist, I think it's more of whatever your preference is in terms of ability to clip in, and aesthetic preferences.