which would be better on or off property?
Ah, yes. The "big" question. If you are budget minded, then you will get more value off site. If you want a "magical" experience(that word is thrown around a lot when discussing staying on site), then on-site would be better where you are immersed in Disney 24/7.
What I mean is that I am able to stay off site in a 1 bedroom suite at a Courtyard by Marriott for under $100 a night using the Entertainment Book discount. The suite has 2 double beds and a pull out sofa bed. They do offer bus service to the parks, but it isn't as frequent as Disney on-site transportation. So the give and take is save money, lose "magic" and a less flexible transportation schedule. I always rent a car, so the transportation isn't a factor for me. And "magic" is over-rated. I get enough of that in the parks. It's nice to get away from it for a while. Even if I factor in the $12 daily parking at the park, I still make out much better.
On-site, you get all the "magic" you want, the use of the Disney Magical Express transportation from and to the airport, use of Disney transportation to the parks and Downtown Disney. Great theming. But you are kind of "stuck" there if you don't have a car. And it will cost more than staying off-site.
As for the fly or drive...I am from the Chicago area and made the drive down once. It was a 2 day drive each way so I had 1 hotel stay on the way down and 1 on the way back. Then the cost of fuel and wear and tear on my car and 2 days of my time Versus spend more, but it is a 2 hour flight and I am there. For me(a family of 4), a no-brainer to fly. Just makes the most sense. Since you would need 6 airfares, that may sway you to drive.
As for places to save...mousesavers.com lists out many deals. And if you do fly and want to rent a car, then they have great car rental discount codes. As stated, the Entertainment book for off-site hotel discounts.
If you do decide to stay on-site, use Kingdom Konsultants. I have only heard good things.
I also should mention the Disney Credit Card. You get Disney points for money you would spend anyways. Those turn into Disney Dollars that you can use just like cash in the parks. You also get a special character meet and greet at Epcot.
