Obviously the Audi looks better value having already depreciated by between £18,000 and £27,860 (assuming no extras, and most are loaded with them). But you need to check exactly what you are getting because Glass's Guide lists no less than 12 different A8 3.0V6 TDI SEs either with front wheel drive and 204PS, or four wheel drive quattro and 250PS. If it’s a 250PS quattro, I'd go for the Audi. The car probably served the first year of its life on Audi's prestige car fleet taking celebrities to premieres, weddings and other events, so will have been kept immaculate.