Chicago (using WoD: Chicago) - Real world city, modified to fit the game setting, plenty of resources and personal experiences to draw on to flavor the setting during game play (possible disadvantage is if the players know more about the "real world" Chicago than what is presented in game and it breaks the mood/scene for them). The book has plenty of interesting NPCs and plots/intrigues to offer if the players get stuck or want to interact with the larger supernatural population.
Newcastle (using Damnation City) - Fiction city with plenty of room for expansion/customization without having to discard any "real world" history. The book has a ton of useful information on city/setting building and WW had provided free maps (but they seem to be missing from RPGNow as of this morning).
All of my players have watched the Dresden Files TV series, but only one of them has read the novels. This game is also going to be a blend of the two settings, not a complete work-up of one or the other. Anyway, what do you think?