Gordon is almost certain to beat Thomson in ERA and WHIP. He'll pick up a few saves, maybe, and some wins. He'll also probably have more K/9, so if your league has an innings limit, he's the better value there, too, though he won't rack up as many in total. So the question comes down to whether Thomson will have enough of an advantage in wins.
In most cases, the top middle relievers are worth more than an average starter.