Somewhere between the two.
Growling or pure vocal?

There's definetely good uses for both- Growling serves well for bands like Testament while pure vocals serve well for bands such as Pantera.

I've noticed that Trivium tends to combine growling and clean vocals well- all of their songs have a combination of both. It's sort of inconvenient when you're trying to introduce a friend to Trivium that doesn't like metal though since there's no clean vocals...

There's really only one right answer to this; it depends on the genre. Clean vocals work better for Power Metal and such, harsh vocals work better for Death/Black Metal, etc.

Both - but nothing beats 5:11 of this song!!!
http://www.youtube.com/watch?v=qVjcD_LPukw
What vocal do you prefer in metal music - pure one or growling?