On one hand, you can destroy the power plant. This makes the opponent pay twice for everything and lose their main defenses(ie: Obelisk and AGT). However if the map is a non defense map, my favorite being volcano, then the decision is even harder.

The refinery, on the other hand, means that they get no 2-credit-per-second bonus, which in itself along with credits for points, can furnish a smart player. They also lose a valuable 300 credit-per-minute(on most maps but longer on maps like city and shorter on maps like island).

For most maps I choose going for the refinery simply for the strategy that after the desruction of the refinery you can sit back and "turtle" as we Ra2 players call it and defend their attacks while piling up points for destroying attacks and repairing buildings and vehicles.

Opinionate this point of view as you please.