I bought multi-shot because I constantly party with a cleric, and because of the clerics' invincible, heal, and whatnot, I can drag an entire mob onto me using the AoE and kill them off.
What skills you buy depends on how you like to play, and same with empowerments.
I like to buy all the skills and the upgrades just in case. =/ and trust me, I've been in a LOT of just in case moments, LOL. Mostly came out of my own stupidity.
For cooldown/sp usage, I chose sp usage cause I like to conserve money >__>" But yeah, it's mainly up to your play style.