Shouldn't the debate be between Sierra Mist vs. 7UP/Sprite? I think one of them went down the toilet in favor of Sierra Mist, but I'm an old timer and switched to the stuff that's bad for your liver, not your teeth
Seeing that Sierra Mist isn't similar to Mt. Dew I really don't see how you could compare them. It would make more sense if you compared Sierra Mist to 7-Up or Sprite.
I don't like how fizzy Sierra Mist is, and I prefer the taste Mountain Dew, so I'll choose that. Also, there are many other flavors of Mountain Dew while Sierra Mist only has one.