I've been struggling with positioning camera in macro photography and have been thinking of getting a macro focusing rail. It seems there are at least two kinds, 2-way and 4-way adjustable.
When should I prefer one to another?
I guess it would depend on what you hope to get out of a focusing rail. Generally speaking, I think a two-way rail is sufficient for the vast majority of macro "focusing" needs, as moving the camera towards or away from the subject at MFD is generally how you focus and compose for macro work.
If you want more capabilities than simply focusing, then a 4-way might be more useful. I've considered a focusing rail system a couple times (have not purchased one yet), and the only useful purpose I could find for a 4-way rail was making composition easier, as I don't really see how panning movements really assist in focusing. I guess if you had a scene with rather complex nuances of depth, or were trying to photograph a bug on the move, the panning ability of a 4-way would probably be very useful.