Maybe I'll fire up either FO3 or NV for the first time soon.
Any suggestion on which one?
I feel NV is the overall better game. Still, here are some things you might want to consider:
Do you want Story, interesting characters, good dialog, good followers and decent weapon customization with a HUGE desert to explore with a lot of different looking areas and settlements that are more populated?
OR
Do you want a decent action flick type of story with less interesting characters, okay dialog, a more oppressive world to explore but with largely same-same areas to visit (think oblivion if you've played that) and no weapon customization?
If the former then play NV, if the latter then play FO3.