Viper & Rebel: you know when you guys post messages next to each other, I stare at your avatars and get semi-hypnotized!
For me it's a draw. I like Red Rock slightly more just because it's newer and has that cool underground section. It's aesthetically more pleasing to me. You can pretty much blast around that track and brakes are used minimally.
Grand Valley offers more in the way of speed changes and difficulties: 2 hairpins, several chicanes, altitude changes...and a long long straight. It's got that 180° tunnel, too.