I've done all three:
Flat-towed my '95 Jeep Wrangler behind a diesel RV and a diesel f350-perfect.
20 foot car trailer behind a diesel F350-perfect.
Tow dolly behind our 2010 Jeep Commander, with a Hemi-perfect.
We're eliminating the car trailer, too much to store. The Jeep is still flat-towed to trail-head, or driven. We use the car dolly for the 924 track car for short and long trips. I wouldn't try to figure out how to flat-tow a P-car though. The tow dolly stores well, and not too expensive buying used. Some people get a bit nervous if you leave the half-shafts installed, and use the 'coast side' of the gears in the diff. Short trips, not a concern.