Between the cost of airfare and the fact that adding ticket says gets so cheap as to you higher, the 9-10 or even 11 really gets us the most bang for our buck. We got ourselves to Florida, we might as well stay a while!
We have taken two of these longer trips with the kids (at age 5 and then 8) and the extra days really let you do everything you wanted to without feeling like it has to be packed in. That said, we rented points both times so we were able to save on the hotel cost, which is probably the biggest downside to long trips. We're also very lucky with jobs and being allowed to take more than a week of vacation at once; I know not everyone has the same opportunity.