I certainly don't want to start a war over this matter. It has been argued consistantly over many years. In fact I argued the side of diffusion sources for many years. That was before I tried both systems.
My results indicate that a well designed condensor system gives not only greater sharpness in the print, it also gives improved local contrast in the print. I think the only way that I became convinced was to try it.
If local contrast and sharpness do not exists to a degree when they are presented to the enlarging lens the enlarging lens has no ability to create what is not there. The argument about the enlarging lens really doesn't have a bearing in this matter so far as I can see. The enlarging lens has no capability to impart a characteristic that does not exist prior to it's involvement in the process. Use a crappy enlarging lens and you will get crappy results with either enlarging system. Use a better quality enlarging lens and you will get better results with either system. The type of light that is passing through the negative equates to "junk in junk out" when it comes to the lens.
So far as I see it based upon results from both enlarging systems in my darkroom the Durst condensor system is far superior to the Saunders VCCE system in terms of sharpness and local contrast. It is also inferior to the Saunders VCCE or an Aristo Cold Light head that I used for years before in terms of apparent dust and scratches on the print. I find that, in my experience, the increased sharpness and local contrast comes at a price.
The question that I never considered in all of my years of arguing diffusion light sources was "why does a diffusion source eliminate dust to a great degree and not effect sharpness to the same degree?" ...as I see it the enlarger or lens has no ability to differentiate between sharpness at one point and not another. If the enlarger light source is washing away the dust then it must be because the light is not passing through the negative as perpendicularly in a diffusion source. If it is washing away the effects of dust on the negative then it must by consequence be washing away local contrast and sharpness. This affects sharpness in my experience, in my darkroom, and on my prints.
I don't know that I accept Ed's explanation for the sole consideration for collimated light sources. That may be one consideration based upon his experience. I find it difficult to accept that this is the only consideration. It is sort of like saying that the only reason for the use of salt in my diet is for electolyte balance in my body. There are other considerations as well, as I see it. For one my food tastes better with salt. Or for that matter it would be akin to saying that automobiles are manufactured for transportation only. That statement is very limited...it takes into account none of the attentuating and coincidental considerations.
However, I think that everyone should use whatever they choose. I honestly hope that more people buy and use diffusion type enlargers. It will make my prints look awfully good by comparison. Good luck.