We have tried all 3 methods:
DCL bus, rental car & private service. It all comes down to convenience and how much you're willing to pay/sacrifice for it. DCL bus is super convenient but when the pick up is at WDW, you sacrifice time on the ship. Rental car is usually cheapest but you sacrifice convenience to save $$. Dragging luggage around, on & off shuttle buses, waiting in lines at the rental car office in PC, waiting in line post cruise for the rental car shuttle, cramming your trunk, tolls, refueling, etc. This is the part we'd rather skip. Not that it's truly difficult but unless the savings is huge or we're on a super-tight
budget, we'd usually rather pay a little extra for convenience.
Also DCL bus doesn't HAVE to be booked as RT. We sometimes just use DCL bus for the return to MCO so we can use their free Onboard Airline check in. Very convenient if you're flying a participating airline. When you put your bags outside your door the last night, you don't see/touch them until you arrive at your home airport. For a 7 night cruise with 2 kids (formal night, pirate night, etc.) it's usually a lot of luggage (compared to a WDW trip) so we really appreciate not dealing with our bags, not having to check them at MCO. Just hop on and off the DCL bus with boarding passes in hand.