We're using a dolly, and I'm not certain to what extent I can recommend the practice. In the past, I've been a strong advocate of dollies, but now think they fill a specialized niche in the RV community.
Yeh, you can haul different vehicles (guests arrive Wednesday with a rented* minivan that'll go on it for our upcoming trip). And, storing it at campsites or event venues has never been a problem. But, the dolly requires special considerations absent in 4-down towing.
The engineering of our Roadmaster is anything but optimum: its Load Range C tires are marginal, and the OEM Carlisle tires -- I dare you to Google "Carlisle" along with "failure" or "dangerous" or "blowout" -- are notoriously dangerous. Two have exploded on ours, destroying the fender each time. I'm using Karrier Loadstars at present, but plan to upgrade both tires to Load Range D models.
However, even if the dolly came with good tires that possessed a large weight range margin, the it must be maintained at regular intervals. The suspension as designed is ridiculously sloppy, making precise alignment impossible. After experiencing uneven wear of my new Loadstars, I've modified the steering arm, and have dialed in the alignment to perfection. It requires a complete re-torque, alignment and lube every 3,000 miles. Not a challenge for even a modest home shop, but daunting to many RV'ers.
Most people gripe about the effort it takes to hook up and unhook the vehicle, but I don't regard that as a problem. Most of the time, I'm loaded or unloaded faster than a 4-down unit. My wife easily mounts the car onto the dolly, has made several solo trips towing her car.
And, dolly use doesn't leave its mark on a car: no excessive mileage, holes in the bumper, wear from the braking unit, etc.
In all, I'd say that dollies are a bad deal unless you meet all three of the following conditions: you wish to tow a variety of vehicles, bending and crouching aren't problems, and you have a shop. If any of those are missing, spend the extra money and give up a little flexibility; tow 4 down.
* All applicable legal and safety precautions have been addressed