Hope you have a great trip! I went at 7 months pregnant (with an eventual 11 pounder) with my DH and 2 1/2 year old DS and we had a great time. The only problem I had with flying was that I couldn't quite get the tray to go flat on the way home - not a big deal, though. Also make sure that you're carrying written permission from your doctor stating that you have his/her permission to fly up until such and such a date. If you don't have that, the airline can refuse to allow you to board the plane. The much shorter plane trip sounds much easier than driving.
While I had a great time and would do it again, I have to admit that it was more tiring than other trips, and it was by far the most leisurely trip we've ever taken. Fortunately my 2yo and I both needed an afternoon nap, so that worked very well. Since he wasn't up to thrill rides, either, he and I sometimes would wait for a character while DH rode a few of the wilder rides.
While I had a great time and would do it again, I have to admit that it was more tiring than other trips, and it was by far the most leisurely trip we've ever taken. Fortunately my 2yo and I both needed an afternoon nap, so that worked very well. Since he wasn't up to thrill rides, either, he and I sometimes would wait for a character while DH rode a few of the wilder rides.