Acctually if you like sonic more dont depend on shadow to be awesome, seeing as its a new idea with a new concept, but in shadow, if your smart enough to stick a second controller in, you can play co-op in the main story! second player is your partner, though you cant do that with eggman dooms eye or charmy though.
and if you need money I advise selling old games you dont want to play, its a good way to make money and get rid of some old games.
and without wi-fi mario kart DS isn't much, wi-fi was its big factor, and without it its just the same experience as any other mario kart.