If your starting a new PvE Elementalist, you'll be best off investing in Fire, at least for the first quater-half of the game. This is because the vast majority of skills you'll get will be fire based, which basically forces you to be a pyromancer.. however as you get through the game you'll start to collect skills from other elements and can mix-and-match your build and even switch over.. but you'll have to wait awhile to be able to do that...