I do not have 3.92 gears and I love my truck. I have never had a complaint about the get up and go of my truck. I don't tow much but the time I had to move it hauled the largest U-haul trailer I could rent up and down the hills of Colorado at freeway speeds just fine. As for mpg I can get 17.5-18mpg regularly taking the back roads to work (2 lane, 40-55mph with stop signs and stop lights). I haven't done a true road trip yet so I don't know the freeway mpg........ yet
I would suggest you get the 730 since there is more support for that radio. I have the 430 (non-nav) and have had to search hard for information and support on it. I also suggest you educate yourself on the Uconnect and Bluetooth add-ons. You can search these boards for more info but the best way to be sure that they are installed and working is to test it with your devices before purchasing.
Hope that helps