I tried a tow dolly first, before going to all four down. You will hear arguments for both. Regardless, I'll not ever go back to a tow dolly if I can help it.
Yes, it may cost less, but be prepared for several issues that go along with it. It was a real pain to load any unload the toad (especially in bad weather). Be prepared to check, recheck after a few miles, and recheck each and every stop. I can not tell you how many times the straps worked loose over a few miles. Repack the bearings every 1K miles. Another set of tires to worry about blowing out....
I never have problems or worries after upgrading to a tow bar and base plate for all four down. It was well worth the cost.
I know, now all the tow dolly enthusiasts will chime in and say why I am wrong, but if you don't take my advice, you can learn the hard way and pay to upgrade with confidence later....