We've done both. For the first season with our Axis we simply rented a car and used that. At the beginning of the 2nd season I picked up the dolly. Now we can take either one of our cars on trips.
On the Axis/Vegas units the mileage hit for towing is pretty negligible. One of the intangibles is that the car is
your car; not some junky rental (of course this depends on how much you like your car LOL). In my case: We can charge up the C-Max at the campground and drive relatively gas free (as long as its within 20 or so miles
).
All of this really depends on why you are camping at your destination? If you are sightseeing and driving a lot then it is worth it to bring a car.