I have personally towed with a tow dolly, tow bar 4 wheels down and an enclosed trailer. I agree with all the comments it is a personal choice which allows you the most flexibility for your circumstances.
I enjoyed towing 4 down, but I couldn't take my Harley or a different vehicle. I spent almost $5k getting my 2004 GMC Denali XL set up to tow 4 down. I spent about $9k for my trailer with the options I thought I wanted to meet my criteria.
I can take any of my 5 vehicles plus my Harley and other items I deem necessary. I can also take multiple motorcycles when we travel for motorcycle trips with other riders.
I have only had to unhook my trailer twice so far at campgrounds, and I haven't had to pay anything extra for storing it onsite. I also have been provided power to my trailer for no additional cost and I have access to park the Harley inside which provides theft and storm damage protection. It always seems to hail in SD around the rally.
I can hook my trailer up by myself, it does take a few minutes longer than 4 down. I load my cars by myself and secure it just as quick as securing the vehicle for 4 down, so in total it is about 15 minutes to hook up the trailer and load the vehicle. Usually I only have to hook the trailer up on departure however. I also do not get wet or any dirtier loading and securing the car in the trailer as I did with 4 down.
I do not have to worry about vandalism to my vehicle when we overnight in parking lots as I have had previous experience with 4 down. My vehicle in the trailer doesn't get road grime, diesel exhaust, chips, etc going down the road as it did 4 down.
I do not currently own any vehicles capable of 4 down towing, not even with modifications, but I do miss the ability to tow 4 down when I need to take the coach to get serviced (usually once per year) as the trailer is truly a hassle to do by yourself if I want to just drop the coach off and pick it up later in the day after the preventative maintenance at Cummins. But that is usually only once per year.
My primary reason to go trailer initially was taking the Harley and vehicle on trips, but now it is necessary as I don't own anything to tow 4 down. I'm not sure even if I had a vehicle to tow 4 down I would go back to it, but if I had a vehicle with 4 down capability and could justify the expense to set up for 4 down again (I lost all of my gear to a fire in 2012 when my Denali caught on fire while towing) it would be nice to have the flexibility.
There are many compromises to all options, I would encourage you to think through all of the possibilities and then go with the one which best suits your circumstances and dollars you are willing to spend. Good Luck!
2009 American Eagle 42'
Trailering HD Road Glide and Saab 9.3 or Cadillac Escalade ESV