Depends how much you want multiplayer.
I rented MW3 from Redbox and beat it in 3 days for $6. That's the best deal I think. If you want multiplayer, pick up Homefront from Gamefly for $8-10. A totally competent clone of CoD multiplayer, with an active community even now. Also includes very fun vehicle combat. A few matches ago, I spent a good while in a helo battle above the map with two other helos. So much going on.