[4x4Poet]

If, by four post you mean you drive the rig onto it, and, by two post you mean you drive the rig between the posts and swing in arms under the frame/body, then:

The two post is best for tire removal, brakes, axle housing swaps, and suspension work, as well as for removing bodies from the frame. The four post is good for driveline work and pretty much everything else. Also, some 4 post lifts can store a vehicle in the up position so another can be driven under to save space.

Edit: I'd consider the 4 post first because it would be larger and require more placement planning. Unless you plan to swap some cabs/bodies, jacks and stands can substitute for a two post fairly well. Speaking within my understanding of 2 & 4 post lifts, of course.