I agree with dbates that there's not much difference between using a dolly and towing 4 down. The most important thing is to get a dolly based on features you want and need. If you consider price alone you're probably going to be disappointed.
We towed our Buick LeSabre on a dolly for 6 years without any problems or hassle. We had a Roadmaster 2000-1 dolly with self loading/storing ramps, electric brakes, built in storage compartments, steerable wheels, and several other features. It isn't the cheapest dolly on the market, but it's easy for 1 person to load and unload. Because of the steerable wheels it follows directly in the path of the rear wheels of the motorhome.
We now tow a Jeep TJ 4 down and there isn't much difference in the time it takes to hook it up and unhook it. The major difference is you need additional storage space at home and at the campground for the dolly.
When we bought the Jeep the original idea was to continue using the dolly to tow it. After reading the owners manual I realized it couldn't be done without risking damage so we purchased a towbar, baseplates, and a braking system to tow it 4 down. The cost of the equipment needed to tow it 4 down was almost exactly the same as the cost of the dolly.
If we purchase another vehicle that couldn't be towed 4 down I wouldn't hesitate for a minute to purchase another quality dolly.