I would guess the wagon has a stiffer rear sway bar because it is rated at a higher load (hence the pneumatic suspension).

However, have a care with stiffening the rear swaybar too much -- it is designed to allow more rear body roll intentionally. This is to introduce some roll oversteer under severe cornering for two reasons: It allows the driver more warning that the car is approaching the tire adhesion limits, and it unloads the outside front tire. On the W115 chassis, with the stiffer roll bars, it is possible to load the outside tire to the point it comes off the rim. Not a pleasant experience. The W123 and W126 chassis (and later ones, I supose) will have the rear end start to come around instead. This is much safer, and easier to recover from (just lift your foot -- this slows the car and decreases the slip angle on the rear tires). It also tends to cause the car to steer more tightly.

This is not a race car, nor a super cornering sports car. For that, get a Mazda RX or something similar. Save the Benz for comfortable high performance highway use.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!