Which is the right move in a split second decision?

Often sway is induced by speed. Jamming the accelerator may result in some "pull" to help get it in line - but it may just exacerbate the problem. Further, unless you have a ton of power on tap your aren't going to have much effect anyway.

Applying the trailer brakes with the lever will almost ALWAYS reduce or eliminate sway if caught soon enough. I think hitting the trailer brakes should be the instant response as soon as sway is noticed.