love my 3.31
I may have different use for my truck than many here, as I will not use it very much for towing. I have a 99 Dodge dually with 24 valve Cummins to tow my RV, so that's not an issue.
I bought this truck as a retirement truck. Maybe some light towing, but more for travel comfort and mileage. I got a rear wheel drive with the 3.31 to help mileage wise, and it has proven to be a good choice. Not many miles on it yet, but about 30.5 mpg combined so far. It is amazing on the highway ... very quiet and plenty of quickness.
I drove a 3.0 with the 3.55 ratio, and it had a bit more off the line quickness, but not really very noticeable. I have heard it will cost 1-2 mpg to get the 3.55, and don't have any info about the top ratio available on these trucks. The biggest mpg cost difference would be the 4x4 vs 2x4.