I've not noticed anything spectacular sales wise. I only have one with each, which was to find out exactly what you are asking, and no I don't think they get better positioning.

I have a fair number of exclusive images at DT and only a couple at Fotolia, but I hope to add more. 

I have said elsewhere on these boards that I don't notice a flood of sales on exclusive images.  However I do think it is worthwhile to upload them for other reasons.  Like supporting sites you like and helping them build better libraries.  Not to mention getting higher royalties when they do sell.