Jeff,
"The tow dolly seems like it would be easier on the van" I can't see a dolly beging easier on your van as you now have more weight to pull. However it is easier on the 850 because the car is simply being carried providing all four wheels are off the ground. Two wheel dollies are used by tow truck drivers when confronted with no choice but to lift a car - opposite the drive wheels. FWD cars then have a dolly under the front and RWD cars have a dolly under the rear. Personally if I was pulling across the country, I would want the car off the ground on a trailer. The towing vehicle would have to be strong enough with all the towing features (tranny cooler etc.) to do the job...