Well, I will be the dissenting voice here. My dd is 3 1/2, and we take her carseat and put her in it on the plane.
I was a flight attendant for 8 years, and watched many videos of actual crashes. In our "recurrent training" that we attended every year, part of the time was spent "reviewing" any findings of the FAA concerning plane crashes that have happened recently. Over and over, we were told that seatbelts on planes were not sufficient to adequately protect people under appx. 50 lbs. No matter how much you tighten them, there is just too much "side to side" movement in the seats to properly secure a child.
I am a VERY protective mother. I feel that God gave me a beautiful child as a gift...and, it is my responsibility to keep her safe for him. That means that I don't take any chances where she is concerned....especially if it is just for my convenience, or to save a little money (I know that is not the case here).
I have been in severe turbulence where I have hit the ceiling of the aircraft with my head, and an ambulance had to be called. It is really scary, and it happens VERY quickly...."clear air turbulence" is the worst, and if you fly into St. Louis in the summer evenings....be very careful. A lot of flight attendants get injured on those flights.
Maybe it is just me, but if I were taking ME (I have never taken ME or Mears for that matter with a child), I would want her in her carseat on the bus as well. Just because car seats are not required on busses does not mean that I wouldn't prefer to use one.
My daughter has a large carseat (it goes up to 80 lbs), and we have never had any problems getting it strapped into the plane. And, because she is so used to the carseat, I think she actually does better on flights with it than she would without it...she falls asleep in her carseat in the car all the time...so it must be pretty comfortable. Plane seats would not give her any place to "lean" to rest and sleep.
We have a Peg Perego Pliko P3 stroller for her which is WONDERFUL in WDW. It collapses like an umbrella stroller, but has a full shade, can recline fully, has a "leg rest" that pops out to support her in a fully reclining position, and comes with a tray and cup holder that also folds umbrella style. It is a GREAT stroller. You can also buy a full rain cover that snaps on for that unpredictable Fla. weather. The great thing about this stroller, is that it folds VERY small, but can easily support her carseat for trips thru the airport. She is always SOO excited at the airport that she wants to walk/run with us, so we use her stroller to hold the carseat, and many other carryon bags. It actually works well. I will be VERY sad when she is too big for strollers!!
You're the parent...do what makes you feel comfortable. But, since you asked, I will give you my advice.
Good luck with whatever you decide, and have fun on your trip!!
Beca