I voted for Dave.
Leno just seems way too rehearsed to me. His Jaywalking bits aren't funny, nor are his game show-type bits. And apparently he has ripped off a lot of ideas from other personalities. I do admit that I tune in on Monday nights whenever possible for Headlines. Though it's not Leno that makes Headlines funny.
Letterman seems to be on the decline, but I'd still prefer him over Leno any night of the week (sauf Headlines). Too bad he doesn't throw stuff on to the street from the roof as much anymore. For some reason, my mother is a fan of Rupert Jee of the Hello Deli.
So given the choices, I'd easily pick Letterman. Though, like many others, I'd rather watch Conan.