Tell him that wagons are not appreciably slower, nor are they bigger. Secondly, and most importantly, they are HATCHES. Instead of a trunk, they have a hatch. I can't understand why people don't *get* the fact that it's the same car. Only thing I wish the wagon had is those great flared fenders.
The biggest selling point for me was that the seats fold down. If you use your car for things other than driving around town this can be important. And you can sleep in it if you have to