I would have to give my vote to the Jarl. As Ben said, Kodlack was (actually, almost all of the Companions were) overrun by a small invading force.
After seeing that, I have absolutely no faith in anything that the Companions say or do, and that includes the words of Kodlack Whitemane. I believe the Jarl would be able to out-fight him and easily slay him if need be.
Well you guys have to take in effect that Kodlak turned into a Werewolf. The Guards would have had to put him down when the Silver Hand attacked. It's a menace, as much as they respect the Harbinger, it's a menace. Werewolves are no different than Vampires. Just though I'd mention that.
Think of it this way Emer. The Circle couldn't turn into Werewolves because the Guards would have to destroy them. Nobody knows why the Silver Hand came to Whiterun. Its just presumed to be a Bandit raid.
The Guards couldn't have defended such a beast. IMO the guards helped the Silver Hand upon realizing their most respected warrior is a beast.