Well the only real exclusive content is the City itself, which has a place to get a haircut from the get-go of the game, a high class furniture and clothes shop, a fortune teller, a ripoff place that has really cool unique items, and a theatre that gives you emotions to display, that and I like it over Wild World because it has a higher average framerate.
Hm, exclusive content? I never got too far into City Folk, so I can't say that I recall any. Can you give me an example? :D (this roughly translates into I never really played City Folk for more than 5 minutes, lol)
Sorry for the really late reply but yes City Folk is my favorite in the series, I think it's mainly due to the fact that I like all of the exclusive content and the fact that I never really played Wild World for more than 5 minutes. But I mean I think that Wild World is really advanced for a DS game! I mostly like it a lot because it's the one I play the most.