I can't help you with Speedplays although a number of my fellow cyclists use them. I've been using Looks for 4 years and have had no knee problems associated with Looks. There is sufficient float to provide plenty of movement and eliminate any twisting issues on the knee.

I like the simplicity and cost of Looks. I opted for the carbon pedal to eliminate "squeeking" from the cleat/shoe/pedal and it works terrifically. No noise, big platform, no hot spotting and simple to get in and out of. My pair of pedals also has an adjustment for tension for getting in and out of the pedal plus an adjustment for moving the pedal further away from the crank. The only downside to mine might be the weight. I have not checked but I'd guess speedplays are lighter than the looks by several grams. I'll probably buy a similar model of Looks if and when these need replacement because I've had good luck with them and it's what I've been accustomed to using.

By the way, I've had some knee pains but they've been due to either my seat being too low or too far forward.

Pedals are one of those personal choices--many models out there to choose from. Good luck with your purchasing decision!!