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If it's just a few cities, it's probably not a big deal either way, and the search results could give you a landing page. If you really plan to expand down the road, I think I'd go with your alternative approach, at least for now. Keep Google on the main site, and you can let users select their appropriate city page (and cookie that). If, down the road, you've got a good structure and solid back-links, and you want to open up the city pages to crawlers, you can always change that later. It's not black-and-white, but in 2012 I think it's best to go out narrow and really focus your link profile on your core pages, expanding once you've got the authority and history to make it work.