Back to Shawn's topic....I've climbed RR first week of Dec and it was ok. Will depend on what weather systems come through. Canyons are colder than in Vegas. JTree should be warmer but no sport climbing unless you consider a single bolt at the crux to be sport.
Fabulous trad climbing of all grades though. You will have potential for snow though in either place that time of year. Will be chilly at night either place. Camping in Vegas sucks...JTree is the nicest place I have ever camped.