The only time I played a wifi game on the DS, it was mario kart, and the guy I played was a hacker. On one race, I had basically won when he suddenly became invincible, ran through me, across the finish line, came back to run over me two more times before finally driving back over the finish line to finish. I was too busy LOLing to be mad.
But, yeah, I'd play wifi. Then I could be one of those guys who has his friend code like 13 times in his signature.