Mechanical rotarys click when you turn them. They require an interface like Druin's to convert each click to a rotation key press that you can hook to an encoder like a keywiz or ipac. I believe the hagstrom encoders have this interface built in to some of their models of encoders.
Optical rotarys turn smoothly more like a spinner. They require a mouse type interface to hook them up to your pc.
So it depends on what games you want to play and how you want them to feel. If you want heavy barrel/ikari warrior like action then you need the mechanical (snk30) style. If you want them to do double duty and be able to use them as a spinner then get the optical rotarys, they'll still work for ikari warriors its just not the same. If your an ikari warrior fan like me you won't like the opticals.