Gonna need a bit more information I'm afraid. What kind of games do you normally play? What are you hoping for from these two games? Seriously, I have no idea what it is that you like in a game, and those two are so wildly different that no-one will be able to give you a proper recommendation.
For what it's worth, I'll be buying Saints Row The Third but I won't be buying Modern Warfare 3. The reason is simply that first-person shooters generally don't appeal to me, particularly those with a war setting, and I absolutely loved Saints Row 2, and I've enjoyed everything that I've seen regarding Saints Row The Third so far.