Which is the right move in a split second decision?

Both methods can work as I have been able to pull trailers out of a sway condition using both methods. The fool proof method way is to ease off the accelerator and apply the trailer brakes. If it's an emergency situation then you can't go wrong with this method.

The mash on the pedal method works, but only if you have the right conditions. Typically it only works well with the lighter trailers and it needs to be pulled by a vehicle that can apply a lot of power to it very quickly. What you are trying to do is apply a sudden jolt to the trailer to get it to snap back in line. Attempting this with heavier trailers, a TV that's under powered, or one that's already near it's max may result in a disaster. In these situation you usually cannot apply the necessary instantaneous jolt. What you end up doing is just building speed gradually which will inevitably make the sway worse. After the trailer is back in line then ease of the gas.