Can't imagine many iMac users will be able to pony up that much for an iMac.
Base Mac Pro/Quad iMac both @ $2800
Mac Pro is the easy decision there.
Tracer
Not necessarily. on top of the price of the mac pro you have to buy a 24" monitor, plus you have all the extra cables and it takes up more space. some people would want that power into a small enclosure because of space considerations and the lack of a need to upgrade.
i know it would be a difficult decision to make if i had to choose between the two. alot of the price of the imac is for it to be such a small simple enclosure.