I agree you need to look at the 35d if you're considering the 4.8. You get the power of the more expensive engine and better MPG than the I6 for about the same price as the I6 with the ECO credit. Less when you consider the $1800 tax credit the diesel brings to the table.

I may be partial here since I have the d, but I'm not sure what the V-8 offers that would make it worth the extra money and crappy mileage other than an exaust note and performance above 90-100mph. I may be strange on here, but Ive been above 100mph on public roads only a handful of times. I just dont think the V-8 is worth it unless you just have to have it for bragging purposes or have money to burn and only want the high end of everything. If you're considering the I6, then I count you out of this last group.