We've been 3 times and always VRBO'd and rented a car. We've also combined Disney with a day at Universal and a beach trip each time. You can easily save money and have more space by staying off of the resort but there are disadvantages. Here are some advantages:
Cheaper lodging. Cheaper food (you can bring some food and drinks into the parks). More space. More privacy. More flexibility.
Here are some advantages to staying on the resort: You don't worry about transporting luggage. Transportation to the parks and within the resort is provided. There will be days when you can enter the parks early.
If you do decide to spend one day at the beach, there should be plenty of car rental places in the Disney World area. One thing to keep in mind about car rentals in Florida is that rental places can't charge you extra for dropping of your car at a different location from where you picked it up.
Cheaper lodging. Cheaper food (you can bring some food and drinks into the parks). More space. More privacy. More flexibility.
Here are some advantages to staying on the resort: You don't worry about transporting luggage. Transportation to the parks and within the resort is provided. There will be days when you can enter the parks early.
If you do decide to spend one day at the beach, there should be plenty of car rental places in the Disney World area. One thing to keep in mind about car rentals in Florida is that rental places can't charge you extra for dropping of your car at a different location from where you picked it up.