One thing that I've not seen in this thread is that with a dolly, you always have an item of value. If you purchase a $1500 dolly, after using it for years, it'll still have quite a bit of value, whereas a 4-down setup is tied to the vehicle that it was purchased for.
I can also use my dolly if I purchase a car at the auction or if a friends car breaks down saving the cost of a tow making it a useful tool outside of RVing.
The dolly has versatility, but 4-down has ease. It's all up to the decision that is right for you.
Tom