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A couple of things re: mileage
-Rear end ratios - there are basically 2 available on 2500 & 3500 GMs - 3.73 and 4.10. Most Duramax's have the 3.73, most 8.1s have (err...had) the 4.10. Check dieselplace.com - they've got a list of all the axle codes and their ratios.
-4WD - the 4x4 versions get 1-2 mpg less, even when running in 2WD mode. That front differential still has to turn, even when it's in neutral. Plus, 4x4s have more stuff to break. Unless you need 4x4 (planning on going off road, live in a blizzard zone, etc.), get a 2WD. 2WDs generally have less resale value, so if you're looking used that's a plus
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From GM, you can *only* get a 3.73 in a Duramax...no choice...on the 8.1L, 3.73 or 4.10 (both of ours are 3.73).
As for the 4x4, if you *never* get off of concrete/asphalt or never see ice/snow, you'll be fine with a 2wd...but I'm here to tell ya, you can get in a pinch EASILY with a trailer in tow...been there, done that. A certain snow covered parking lot in Nebraska comes to mind...
