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.
Trueshot Aura also increases your pet damage. You get a better pet without having to spend a talent point in the Beast Mastery tree. Not bad, eh?
Trueshot Aura increases the damage of your melee-based party members, or even the ranged attack power of another Hunter who might be in your group.
Perhaps best of all, when you invest in a 31 point talent, you can only get one 21 point talent -- the one that's in the same tree. And the 21 point talent in the Marksmanship tree is Scattershot, the one talent I find it hard to live without in PvP settings.
Bestial Wrath you can use for 15 seconds once every two minutes, as long as your pet is alive. Wyvern Sting you can only use out of combat once every two minutes. With Trueshot Aura you get an ability that is always on and always modifies your damage, makes your pet more dangerous, and enhances your party. And you still get Scattershot to boot.
When it comes to top tier Hunter talents, I declare a winner...Trueshot Aura!