Fire 9 is the ONLY way to give your regular melee troops magic weapons in the entire game, so it wins.
Especially given the Moloch Cost effectiveness.
Water 9 is incredibly synergistic, but there's no template with both paths, so it's horrifically expensive.
The only serious MP usable 9/9 is fire/astral using Bahomet for Marignon, and even that gets a little crazy in terms of scale sacrifices.
Jotuns with w9/n9 or hydras with fire9/w9 sure are fun in SP, though.
Rabe