I agree that Scrivener search sometimes seems to be slightly faster than the Finder, for whatever reason, but not so much that it really improved search results for me. Along with the Finder keeping my research more easily accessible to other document-making apps like iAWriter and Pages, I just saw no reason to add another layer of organization to my research docs when the Finder could do the job just as well at ground level. But please do let us know if it works better for you; I’m happy to reconsider.
Oh, and if Spotlight isn’t working for you, there are other alternatives, like Alfred and EasyFind.