Both can regenerate from ridiculous wounds, but it actually taxes the Surfer to do so. Loki exerted no energy in restoring himself after being decapitated. The Surfer on the other hand spent a lot of energy restoring himself after he was diced up by that mainframe chick in V3. There also the matter of energy attacks being turned against the Surfer. If he were to attack Loki with it, Loki would be able to wield the power cosmic as well.