Shel, I think (perhaps wrongly) that when you take a regular lens and put it on extender, bellows or any other such device, you essentially enlarge the middle portion of the image circle thereby achieving your magnification.
The macro lens is designed to sustain such an "abuse" and still give you excellent resolution. So, from resolution point of view, it seems macro lens would prevail. And it seems to me that in macro shooting, resolution is quite important factor... Personally, I've found that working with converters wasn't good enough from resolution point of view, while working with extension tubes was too cumbersome on the field. So I bought a macro lens, and I am very happy about this very purchase. -- Boris
