The original SPD is the usual MTB style with a 2 bolt mounting system. SPD-SL is a 3 bolt style that's most popular with road shoes. Look and Time (among others) will attach directly to the 3-bolt pattern and Speedplays come with an adapter plate that allows you to attach them to the 3 bolt pattern. Crank brothers also makes an adapter plate that allows you to mount 2-bolt SPD cleats to 3-bolt SPD-SL shoes.
Personally, I like speedplays because I've disintegrated a majority of the lateral-collateral ligaments in both of my ankles. If I use pedals with no or re-centering float, they cause me to have ankle discomfort (it feels like the resistance holding my foot straight is also causing my anklebones to separate).
I've got the Speedplay x5 now, and I'm planning on upgrading to the X2 or 1 in the near future
Oh yeah, and I've never had an issue with them disengaging when they shouldn't, and I've never had a problem getting in/out when I want to



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