My son is 2 1/2 as well and we'll be traveling in April / May to Disney. On our previous trips (when he was 1, 1 1/2, and 2) we always used a car seat on the plane. We actually specifically purchased this car seat to use for travel, and also use it in my husband's car, because it's very lightweight. The big thing with car seats is to make sure it will fit in the seat on the plane. Most plane seats are very narrow, so not all car seats will fit in them. Measure the width of the base of your seat and then go to your airline's website. There should be a page there that tells you how wide the seat can be. If not, just call your airline. Also, the seat must be FAA certified. That info should be printed somewhere on your seat, probably on the sticker that has all the safety info on it.
One problem I have run into is that the buckle of the seat belt on the plan often ends up right in the middle of my son's back, with only a small bit of fabric inbetween him and the buckle. To remedy this, we just twist the male end of the seat belt to make it shorter. Then the buckle doesn't end up positioned in the middle of his back.
My son is used to being in a car seat for travel, so it just makes things a lot easier to have him strapped in. My guess is, if he were just in the seat with the regular airline buckle, he'd be able to unlatch himself, which would be a little too stressful for me. The other nice thing is that if he falls asleep, he can just rest his head on the side of the car seat, so he's pretty comfortable.