To be honest, neither of them appeal to me. Like Tets said, there's no point in buying them until some good, quality titles are announced. Sonic DS isn't exactly a game, it was a demo shown at E3 to demonstrate how the second screen could be used. And why would anyone care? The object of the game is to rub your finger on the second screen to make sonic run faster. Sonic Team did some crazy stuff before, but I know they're smart enough to realize what a mistake that would be. Truth be told, the second screen is basically just a gimmick to earn some extra cash. And being the money-grubbing whores that they are, you'll probably have to use it to connect to their next console via link cable for "extra goodies". But on the other hand, This is Sony's first handheld, so they're inexperienced, which gives Nintendo the advantage. So until I see something really outstanding, my money is staying in my pocket.
-End rant-
-End rant-