As Moonkin do you go for +damage or +nature damage gear? So far I'm +damage in general but when I'm grinding I'm usually doing IS + Wrath spam. When I'm in an instance I usually do IS+FF+SF so Arcane damage might be an improvement for my DPS but I'm not sure.

The best/most accessible moonkin gear tends not to focus on +nature or +arcane, you can use these pieces to supplement it if you choose, but i usually go for the best all around upgrade (stats wise) to a slot, its really a matter of play style.

I gear with +spelldamage as I'm never sticking to just 1 school in any of my rotations.
However, I do use flask of blinding light instead of the supreme power, but the other way around doesnt make sense ;)