I'm actually debating on whether to drive or fly also! We just went to Disney in May and flew. We have always wanted to experience a road trip and 3 out of our 4 kids do as well. There are ups and downs to each, I'm just going to wait it out and see what I end up deciding on when it comes down to it. We are going in September, so I figure I have a month or two to decide?
I find the ideal time to book airfare is 2-3 months in advance. I like using the "flexible date search" on Travelocity to see how far in advance the airline is offering its lowest fares (look at the 'valid through' dates for the lowest fares).