Sedo has more sales, but also domains listed for sale there.
At Afternic you have a greater chance of being "featured" for free.
You can, of course, list for sale at both, but I understand your desire to park where you get the most sales.
You can have a hosted site with the afternic or sedo "For Sale" page as the base page, with a popup/popunder (Or Framed?) page with the "FOR SALE" page for the other service in the background. You can popup/popunder the revenue generating parked pages, but the "For Sale" pages are a different animal entirely, afaik.
At this point, I like the Afternic people better (I think), but if my goal really is sales... I'd have to go Sedo.
-Allan