The OM-3 and OM-4 have an averaging function that lets you take a bunch of meter readings and average them together. The OM-2SP has a spot meter that takes one reading. The reason I like the 4 and 2SP is that they both show you the actual shutter speed instead of just arrows or lines.
I have the OM-2SP and the OM-4. I like the SP for people. I dial in +1 (for light skinned people) and then meter their face. Great exposure every time, and fast. The averaging function on the 4 isn't easy to learn, but I imagine it can be very powerful.