The Vauxhall has a higher base PP, the Opel may have a slight disadvantage in front mechanical grip from my trials finding fun 500 PP race cars earlier this year. The Vauxhall seemed to have a good 1/2 second a lap over the Opel at GV when tuned identical despite enjoying a little extra power from the lower PP.
Many doppleganger cars in the game have subtle to major driving differences if you give them a chance.