The difference between stupidity and ignorance is that ignorance is curable. Sounds like this guy is hopelessly stupid. What outrageous misstatements! I hate to admit it but my anger is greater than my amusement.
There may be cars that outperform the Benz, but an '89 Caddie Deville is not one of them. It's overweight, undersprung and poorly balanced. The front end will plow like a snow plow. Although the STS is pretty nice, I prefer the engineering, quality and feel of a Mercedes. One should, as they are significantly more expensive (V8 to V8). I feel the Deville is comparable to the S class for size and V8 power. Compare a 300E to the Caddy Cetera HAHAHA...
But since the guy is stupid, you cannot change his mind. If the facts don't do it, what will? Another old saying for you to use (if he won't knife, shoot or beat you up for it): You can't teach a pig to sing, it only wastes your time and irritates the pig. Good luck.

Steve
'85 500SL (Euro)