Western cred? If you don't consider No Country for Old Men at least Western influenced, you weren't paying attention.
Also, Viggo has no bankability. Seriously, the only movies of his that were big hits were LOTR, and he was a supporting character in those. Appaloosa, Alatriste, Eastern Promises, A History of Violence, and Hidaldgo didn't set the box office on fire. Maybe The Road will change that, but there's no evidence that his name brings out people in substantial numbers.
Both are good actors. Neither is a box office draw. Heck, who says Viggo is even interested?
By the way, I don't think anyone is suggesting that Jonah Hex cost $100 million and open in the middle of Summer. You don't need high priced actors if you keep the budget reasonable and pick a good opening date, like March 2010. I could certainly see it marketed as a followup to 300 and Watchmen if WB marketing is on the ball.
Edit: Heck, why are we even concerned about how much bank Jonah Hex will make? Do you have stock in WB? I'm much more concerned about whether it's a good movie and Josh Brolin should do a fine job in the role. Let the accountants worry about the rest.