Excellent point, and one which I thought about. I think that the Hulk, (once blasted by energy that disintegrated 90% of his form and he still regenerated), would indeed die if completely ripped apart, as opposed to simply being reduced to less of what he is, albeit intact.
I think that Doomsday's regeneration is a bit stronger, requiring him to be completely destroyed without a trace to prevent regeneration, such as the time on Hunter and Prey, when he was taken to the end of time, when all was entropy, and was completely enveloped and destroyed, leaving nothing that could come back.
If not, then, they fight forever, killing each other add nauseum then regenerating.
Sheesh! - Talk about boring sequels!!!!
I just can't stand Doomsday, so whenever you pit anyone against him, my vote will go to the one that ain't Doomsday. 
I generally like to use reason, not just preference for a character to chose who I think would win, but Doomsday is the exception for me as well.
