There are pros and cons to each.
Trailer:
Pro:
Any car is capable of being hauled without modification.
Con:
Cost of purchasing trailer.
Cost of upkeep, ie tires, bearing maintenance, license fees
Weight of trailer reduces the size (weight) of the car you can car. If the max towing capacity of your motorhome is 5,000 lbs, and the trailer itself weighs 2,000 then you are limited to a car of 3,000 or less.
Takes most amount of time/effort to load and tie down car onto trailer.
Need to deal with storage of trailer, both at home and at campsites.
Dolly:
Pro: Most, if not all, front wheel drive cars can be hauled without modification.
Cons:
Essentially same list as above, but to a lesser extent.
Tow Bar:
Pro:
Easiest to hook up.
Least maintenance
No storage issues.
Cons:
Not all cars are 4 down towable.
Some may be made towable by installing extra equipment, such as driveshaft disconnects or transmission lube pumps.
A base plate must be installed on the towed car, providing the brackets to attach the tow bar.
Cost of adding base plates and lube pumps. These items are specific to the car on which they are installed and usually are not transferable to other cars.
These are items that come to mind. I am sure that others may have others to add.
There is no right or wrong answer. I comes down to your personal needs and preferences. I currently tow 4 down.
