I think one of the defining characteristics of urban fantasy, as compared with traditional fantasy, is precisely that its setting is modern-day Earth. If you go with an alternative Earth or an Earth-like world, you're pretty much venturing into the realm of more traditional fantasy.
One option might be to set it in the near future, say 20-50 years ahead of now. You can give the players some gear options that wouldn't otherwise be available, and it also allows you to change present-day Earth to suit your needs. You can have a grim and gritty Bladerunner-type setting, or a more utopian world, whatever you want.
For example (not urban fantasy, but just to illustrate some possibilities), I set a long-running d20 Modern campaign 100 years in the future, in a totalitarian world resulting from global war and diminishing resources, and added a small degree of technological advancement (mostly weapons). Then threw in some mutants and demons and planar travel (angels in tanks!) for fun.