Differences among tow dollys:
1. Brakes: None vs Surge (drum or disc) vs electric drum
2. Pivoting wheel rest vs fixed.
3. Max vehicle width
4. Max weight capacity (but remember only the front of the car is on the dolly)
I strongly recommend some type of dolly brakes, but don't think the type is a significant factor. However, any surge type has the advantage of not requiring an electric brake controller installed on the motorhome.
I'm a fan of the pivoting wheel table (where the car wheels rest) but its probably not a big deal. With a pivoting table, the car steering must be immobilized (locked); with a fixed table the car steering must be free to turn as needed. There is a small difference in convenience if the car you tow has or does not have a locking steering column.
Obviously the width has to be sufficient for the vehicles you tow. And the weight capacity as well.