Done both. Started with a Tandemtow with the motorcycle on it and an Escape. Ratchets wouldn't allow the VW Beetle to be used. Replaced that dolly with a KarKaddy SS and it worked well. Easy to tow, costs was around $1700 bought used and that includes the new tires I put on it. Decided I wanted to flat tow so went the Wrangler route with all the appropriate towing and safety gear. Again easy to tow and easier to hookup and unhook.
When connected, either the car on dolly or flat-tow, backing is impossible. With a dolly a very short and straight is possible such as overshooting the hookups in a pull-thru site but with flat tow you risk permanent damage to the tow bar attempting that. With the car removed from the dolly, backing is possible and easy if you are reasonably talented with backing a trailer. Some dollies add considerable tongue weight and can't be turned as sharp as a good flat tow. Connecting/disconnecting in weather with wet/muddy/frozen straps ain't fun. More cars can be dolly towed than flat.
Costs: Towing with a dolly is far less expensive especially getting a used quality dolly. The cost of setting up a vehicle for flat towing can be considerable especially if you contract for everything to be installed, easily over 4K. Bottom line is what works best for you. If you do a lot of snow birding and stay places long time, a dolly might be a good choice. If you like us move every few days, flat might be better. Other posters give their insight and opinions based on some other factors. All good reading. When you read a poster who ridicules someone's decision and style, skip that post and realize they probably are not happy people.
2007 Itasca Horizon 40FD, 2015 Harley Davidson FLHTKL, 2012 Jeep Wrangler Sahara