Yes a rental car is definitely cheaper, and gives you a lot of flexiblity as well. That's what we did last month. We got an 8-day rental from National (full-size for $143), used it to go to WDW, drive around there during our stay, drive to Port Canaveral, where we parked 3 days for $21, and then after the cruise spent another day visiting relatives. We left our resort at 10:30 on cruise day and arrived well before the Disneys buses. Using Disney transportation would have cost us (as you know) $250.
Actually it's possible to drop off your rental car before the cruise and get another afterward for further savings, but I decided to "splurge" and avoid that inconvenience.
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