There will be an MPG difference, albeit a very small one. If you don't tow very often, and don't plan to level/lift and get bigger tires, the 3.55 should be a nice highway driver.
If you are considering the max tow package, then you probably do plan to tow at some point, I would consider the 3.73. I have those coupled with the 5.0 v8, and it is a great town driver, and still gets 20-21mpg on the highway (flat, 65mph).
Best of luck!
Last edited by cwrose; 07-18-2012 at 12:15 PM.