We generally won't even consider flying unless it's more than 6-8 hours (or if we get a really good fare). With 5 of us, those airline tickets add up, and then when you add parking (or transportation to the airport) and getting a rental car at the destination (especially on longer trips), it's far cheaper to drive for most domestic vacations. Plus, that "2 hour flight" is really more like 6+ hours when you factor in the drive to the airport, getting there early enough to make it park, get to the terminal, check bags, get through security, and get to the gate with plenty of time to spare, seemingly inevitable delays, getting bags at the destination, shuttling to the rental car center, and driving to the timeshare. Within 6 hours, driving is just as fast, and you can leave when you want to.
If it's more than about 12 hours away (the most we would drive in a day), we start looking much more seriously at flying. At 16-24 hours (2+ days of driving each way), we'll generally only drive if it's a very long trip (we've taken several 4-6 week trips).
It's also SO much more convenient to have your own vehicle. Especially with kids.