Using iPhone in France

I will be going to France later this year and wanted to be able to use GPS and offline maps on the iPhone for navigation on streets and the Metro while walking around (I have a 3G ATT iPad, but that is too bulky to carry around while walking).

I do not plan to use the iPhone for making phone calls while in France.

Given my interest mainly in GPS and navigation in France and not in making phone calls there, does it matter whether I purchase the iPhone from ATT or Verizon. I know the Verizon 3G does not work in France, but not sure if that is relevant to my plans.

I would prefer Verizon as that is my provider now.

Thanks for your advice.