Since you are looking for a mega cab you have to buy an 06 or early 07 to get a 5.9. Mileage will vary depending on how fast you drive and how heavy of a load you are pulling. On a stock truck Id expect 12 to 14 towing and 15 to 20 mpg empty. This is actual mileage and not what the lie-o-meter says on the overhead. A 6.7 will get less mileage than a 5.9 in stock form due to the emissions crap on them. If you dont have emmissions testing in your area and end up with a 6.7, by all means delete the DPF and EGR. You will not regret it. Mileage will jump to 5.9 levels or better. I own a 6.7 4x4 6spd after owning 5 previous 5.9s and have no complaints with it. The first thing I did was remove all the emmisions garb. Installed a H&S tuner and it gets great mileage with plenty of pulling power. The only downside is the initial cost of the the tuner. You will need a Smarty, H&S XRT pro, or EFI live to make the truck function properly with the emmissions crap removed. The tuners have went up in price significantly in the last year or so. As for the 4x2 vs. 4x4, Id go with the 4x4 for resale reasons, and secondly because a 2wd diesel pickup will get stuck on level ground if its the least bit slick. They are helpless. .