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This answer mentions that Shimano makes two kinds of SPD cleats, one that allows unidirectional release (SH51) and one multidirectional (SH56).
Are there other differences between the two? Will both types work with all SPD-compatible pedals? Are there reasons to prefer unidirectional release over multidirectional release?
Yes, all SPD pedals are compatible with both of these cleats. The multi-directional release SH-56 will allow you to pull out of the pedal vertically if it is required. They are designed for spin bikes. (Exercise use.)
The difference is a preference only, because it depends what you intend to use them for. I will not use the SH-56 on the road because I have pulled out of the pedal during normal riding, and offroad has similar issues.
There are 2 kinds of "SPD".
There's the old/original style, which today is more often used on mountainbikes and recreational riding, and the newer type, which is a very large triangular plate, used predominantly on road bikes (racing community, enthusiasts, and sunday cyclists who have the means to buy the high-end stuff but don't really need it).
Personally, I prefer the original (traditional) SPD type cleats, even for my track and road bikes. I use mountainbike-type shoes, that allow me to walk around, run my errands, without the clip-clap and the skating all over the floor.
Within the original type, there are 3 models: SH51, SH52, SH56.
From what I could gather,
SH51 requires pivoting your foot horizontally to disengage, but according to a youtube video I saw today, will also disengage if your foot rolls (such as in the natural instinctive movement when you want to remove your foot to the side, or during a fall)...
SH56 disengages either by pivoting horizontally, or pointing your toe downward and pulling up, or twisting to either side. The SH56 is the most beginner-friendly.
Later was introduced the SH52. this one holds on a bit tighter than the SH51, and isn't as prone to releasing if you roll your foot to the side. This is more for expert mountainbikers, who don't want the foot to disengage when they're doing advanced stuff, or expert road cyclists, who don't like that the cleats disengage too easily during hard acceleration, or steep uphills.
