as mentioned, the differences are: perspective (very interesting demonstration above, talking about background, i guess that's half of it when you talk about macro
); workign distance (very important for "disturbable" subjects). a very good point is which you would be more likely to use for things other than macro (i favour longer lenses, so it would be the 100 for me, it might not be for you, though). regarding dof, i tend to disagree, if i remember correctly, for macro purposes (close to 1/1 magnification), the magnification of the subject and the aperture will determine the dof, so actually a longer lens will allow you better dof, because you will be able to stop down further before difraction is a problem (however, i have yet to do "scientific" experiments on this subject
).
albert: i feel your pain, i have shot a 200mm on extension tubes, i loved it (though it is unconfortable, and of course, a dedicated macro lens would be better optically too, for macro). i see sigma is making a 180 macro, it's not cheap, but at least it's not ridiculous as the pentax 200 macro second hand.. (and i hear both sigma and tamron make excellent macro lenses), hope this helps a bit.
ps: why would you ever reverse a macro lens? reversing for macro only makes sense for "normal" lenses, as far as i understand, to try to get around the disadvantages of the "1 to many" optical design of a regular lens (turning it into many to one, which somehow seems to work better for macro)? or am i wrong?