I think eBay has a few scales, Amazon has lots of them! It's a weighty issue for sure... Really though, it's easy enough to set up both of them, don't limit yourself to just one... Keyword = DIVERSIFY! ECS Dave
I think eBay is better as they pay $25 for every new user you get for them (must register and make a bid).
I have heard that Amazon is pretty aggressive when it comes to analyzing what sells via all channels, and making sure that they're acquiring/pricing their own inventory to maximize profits, even if it means screwing other sellers. Ebay, of course, has no incentive to do this, since they don't sell anything themselves. However, for finding shoppers just prior to the point of purchase, I'd have to think Amazon wins. (Although have heard that Ebay's soon-to-launch redesign will dramatically improve their usability.)
You can sell a lot more volume on ebay but you will probably sell for a higher price on Amazon. So if you only have a couple of an item, sell on Amazon. If you have a large amount of something or unlimited quantities focus more on ebay.