More attack power means more base damage on the Hunter's Aimed Shots, Multishots, and Autoshots. (Attack power doesn't modify Arcane Shot or Serpent Sting.) This feeds into larger critical strikes too.
Trueshot Aura has the benefit of being a passive ability. Once you flip it on, you don't have to worry about it. It's just there, doing its damage-enhancing goodness all the time.