The 8100 is a great motor, and does pull very well.
The Duramax is $4000 more when you buy, and yes Diesel is slightly more around here. $2.95 for regular Vs $2.97 for Diesel. So the argument is always, "sure you'll get better MPG, but look how long it will take to recoup that money." That's true, however when you go to sell the Diesel motor will make the resale value more than $4000 higher than the 8100 or the 6.0 motor, so it's a win- win, if you can swing it at purchase time.
Like I said one of my racing buddies has an 02 2500HD with the 8100 and he loves it. He swore up and down that he could tow right with me (he has a 24 ft, mine is a 28) but the first uphill on ramp I put over a 1/4 mile on him... He never said anything about it again...
The 6.0 is also a Great motor with a ton of aftermarket parts available.