Diesels get better mpg and if you were to get the V10 magnum you'd be paying just as much for gas.

Here's the years for both V10s

Magnum V10
1994-2001 and 2003

SRT10 Viper
2004-2006

 

The mpg's of diesel over gas are minimal, especially now with all the emissions restrictions. 10mpg with any of the current diesels from Ford, Chevy and Dodge is the norm. My 6.0 powerstroke towed my travel trailer and gear and only got 8mpg. My wifes, Grand Cherokee did better than that. Obviously I had to keep the speed down on the Jeep due to its size, but still.

When you factor in the oil changes, fuel filters, and cost of diesel fuel you will never even come close to breaking even. Add in how high a diesel engine repair is and you are in a DEEP hole. Unless you're towing well over 10K on a very regular basis a diesel is a waste of money.