As Mark stated, there is little or no perceivable difference in the driving experience between a 2008 and a 2009 Viper. Although the engine software and calibration is different for regulatory reasons and refinement, most customers will never notice a difference.
Some customers have asked if they can install the 2009 calibration in their 2008 vehicles. The answer is no, because the 2009 calibration is not certified for use in 2008 vehicles.
Note that the MOPAR calibration is based off the 2009 production calibration, but there are significant differences that WILL be perceived by the customer.
The fuel tank and the rear frame changed slightly between 2008 and 2009 to meet new federal rear impact requirements. However, these changes are minor and add less than three pounds to the weight of the vehicle. There are no other significant weight changes to the vehicle.
There was one change in mid-model year 2008 (roughly mid-April 2008) that affected weight. We upgraded to solid halfshafts on the Viper ACR, which were required due to the added grip. For the sake of simplicity at the assembly plant, that change was made to all Vipers at that time, and added about 6 lbs per vehicle.
Also, there is a new optional 6 spoke wheel available for 2009. These wheels are roughly the same weight as the Viper ACR Sidewinder wheels, saving approximately 30 lbs per vehicle over the standard 5 spoke wheels.
The bottom line is that you should select your Viper based on your preferences for color, wheels, and other options. Both model years are great cars!