One thing to keep in mind is rotational movement on the pedal or float.
On my road bike, I use SPD-SL yellow cleats on the bottoms of my shoes because they have 6 degress of float. There are also red and blue with 0 and 2 degrees of float.
If you have a straight pedal gait, for lack of a better word, you can get by with the 0 and 2 degree float cleats. If these are not adjusted right, you can get some knee pain due to forcing your knees one way or the other.

Some cleat/clipless systems have adjustable float.

Now on my mountain bike, I use crank brothers egg beaters and they seem to have a lot of float. I like them as they are easy to unclip when you need to quickly get off of the bike, hold tight on the upstroke and can be clipped in by pushing front or back.