Well that is absolutely your choice, if you're travelling ALOT like alteast 1 day a week and it takes quite a while and you're bored, get it for the DS, if you're @ home anyway, the Wii has way better performance than the DS so the gameplay would be better aswell.
mayb we can add another suggestion, y dont u buy 4 both if u really like it?? U helped on funding the makers to make another version, and u can also choose which 1 u wanna play :P