I have both. I bought the Marathon for my first and then the Roundabout for my second. Both my girls are small and at 4-1/2 my oldest outgrew the Marathon and is now in a booster seat and my second daughter was right at the border line for height in the Roundabout at three years, she is now in the Marathon and my one year old son is in the Roundabout. I have never flown with either carseat, but the Marathon is huge. The one time we did fly, I bought a smaller carseat for the plane. If I remember correctly the Marathon does not fit in the plane seats, but you can check with the airlines. I have to say, I love both of the car seats. If your girls are on the bigger side, you might want to get the Marathon.