We've never flown, so I don't know how much plane tickets cost. But the bigger your family is, I would think the more economical it would be to drive (as long as they can all fit in one vehicle, that is). Say you are a family of 4, like us. You have to buy 4 plane tickets. Again, I'm not sure how much a plane ticket would be, but I do know that you are buying 4 plane tickets, whereas your gas for the van costs the same whether you are driving alone, or with three other people.
We drive from Illinois, and in those first couple years when my wife and I drove straight through, it took us around 18 hours. Now we stop once or twice on the way down. For me, the drive is part of the experience. I enjoy seeing the sights, hearing the sounds of the DVDs playing in back, maybe finding a good place to stop and eat along the way (for us, this is usually on the way back, because on the way there, I want to make the best possible time).
Ultimately, if you are only concerned about the economics of it, just sit down and count up the expenses. As I said, for each person in the party, you have to buy a plane ticket, but your fuel costs are the same no matter how many people are in your vehicle. But then, on top of that, you have to factor in how many people you are feeding and how much it will cost to feed them when you stop along the way to eat. Will you stop for breakfast, lunch and supper, or just eat out of the cooler for most of the trip and only stop for one meal? Are you driving straight through, or stopping at a hotel? The question will be, do you think your fuel costs, food costs and lodging costs will be smaller or bigger than plane ticket costs?
15 hours one way ... not bad.
