Due to all the modern engine complexity, hpfp and others, and increased engine output, I would be very careful to say either way which would last longer. I think all modern car engines' longevity pale in comparison to their forebears.

Your particular vehicle with its particular set of circumstances would likely be a larger determining factor in determining longevity.

For instance, if you do alot of short trips (which isn't good in general) the V8 would be preferable to the diesel since it takes longer for the diesel to reach operating temperature and burn off the condensation in the oil.

Do you have an excellent fuel supplier for gas (low to no ethanol good qc) or diesel (high cetane, no contamination, high turnover)?

Are you an aggressive driver that demands fast acceleration from you car, or are you content to cruise with the torque?

As others have said, pick the one you like the most and if it doesn't last quite as long, that'll be ok too...

Reminds me of my purchase process in buying the 35d. I was intially torn between the ML320CDI and the bimmer since the benz engine uses a duplex timing chain and the BMW uses a simplex. In the end I liked the way the 35d drove better, so I bought it.

Enjoy your life and your purchases and don't worry too much about absolute longevity.