I like them both - A LOT. But I've been a big Tower of Terror fan since it opened, and I always will be. Tower of Terror showcases what Walt Disney Imagineering can do when they really do it right better than Rock 'n' Roller Coaster.
I voted for RnR cuz ToT uses suspense and the drops are gut-wrenching, but there's something about RnR that keeps you going throughout the ENTIRE ride, the music, atmosphere, I dunno... It just ROCKS! (Maybe you just have to ride it with an INTENSE Aerosmith Fan, like my best friend, it made it that much more exciting)