Did you purchase a ticket for your daughter? I know, most airlines say over two must purchase a ticket, but we know some people don't...
The only reason I ask is that if you did not, you will only be able to use the car seat if there is an empty seat next to you.
But, assuming you did purchase a ticket for her, I agree with everyone else, bring the car seat. Its a pain to lug around, but it is infinitely safer for a child than the lap belt on the plane. Also, as somebody else said, it makes them feel like they are in the car, where they know they have to stay put.
Make sure you ask to pre-board, as it is much easier to get the car seat down the aisle while the seats are empty. Just in case the flight attendant does not mention it, put the car seat by the window. If you are in a center section, the car seat should go on the inside. This is so in the event of an emergency, the car seat does not block anyone's exit.
Also, when deboarding, I highly recommend waiting until everyone behind you has walked by. Again, its much easier to get a car seat down the aisle with no people around.