Reno is outstanding for moutainous twisties (think Sierra Nevada mountains, like Fred said). It's also close to Virginia City, which has some spectacular desert scenery twisties. Also, good access to fun routes like Gerlach/Pyramid Lake...straight desert but beautiful.
Las Vegas is more straight desert roads. The Valley of Fire State Park is nearby, with some gorgeous twisties and truly jawdropping rock formations. Coming back into California has Death Valley State Park very close...awesome riding there, especially now that they're re-opened most of the roads after the big flood there a couple of years ago. Really great off-roading, too, if you're into that. Las Vegas is also closer to Arizona, which has some great twisties after you spend a day crossing the desert to get there.
The Prescott/Sedona area in particular is very nice.
I'd honestly be hard pressed to choose between the two, if all else were equal. I think most people would go Reno in a heartbeat, but I really love the desert.
Eh, I'd probably still choose Reno. They're really only one long day of riding apart, anyway.
My photos from between Reno and Las Vegas last September
