We are comparing the Audi A8 and the Mercedes S-Class to determine which is the best luxurious plug-in hybrid. Both cars are top of the line luxury vehicles, leaving the question of which one is superior. The Audi A8 has a 3-liter V6 turbo engine with an electric motor that delivers 462hp, while the Mercedes S-Class has a 3-liter straight-six engine with an EV motor delivering 367hp and 150hp, with no combined figure available. The Audi A8 is priced at £86,000, while the long-wheelbase version starts at £89,000. On the other hand, the S-Class LWB edition is priced at least £111,000. There is a substantial price difference between the two.