I think most would agree that diesel can get better MPG and has more low end grunt. The bottom line as I see it is this, the upfront cost for the Duramax/Allison combo with a typical annual mileage of 12k per year would take you several years (more than 5 I think) to save enough better MPG which would translate into cost savings to offset the price of the upgrade in engine and trans. Of course the more miles you drive, the faster the payback happens. As diesel prices rise however, it will no doubt take far long for the return on investment...add the fact that the vehicle is depreciating each letter of this post I type (though will have a higher resale later depending on several factors-- one of which is the rebates which is starting to hurt resale).
This is not a bash on the gassers nor the diesels, but the facts are clear. You will loose either way you go, it just depends how fast. At the end of it's life, it's still a $35-$40k rust bucket and a sunk cost as they say in accounting.
