We're a family of seven (six over the age of 20) so I've been considering some of my options too. I can get two rooms at a value (with a Disney Visa discount) for around 60.00/night per room. This gives us two bathrooms, but no cooking facilities. You have too many guests for the homes at Fort Wilderness. I'm going to consider renting points from a DVC member for a 2 br villa. I should be able to get that for around 200.00/night.
The only thought I have about staying offsite is we actually 'want' to have disney transportation. Many times one of the kids was feeling bad, or DH was tired, or for whatever reason - some of our group decided to go back to the room while the others wanted to stay. If we'd stayed offsite, this wouldn't have been available. And -no- I was not going to rent two cars!
Have fun!