We would bring our own car seat at that age.
I have heard good things about the CARES harness but only within a certain weight/height range. I have heard it is not great for smaller kids.
If you do bring your own seat make sure it is approved for air travel. I would also print out a sheet with info on our rights to use the seat on a plane. We never had any issues but I have heard of people having problems. You will have to put the car seat in the window seat if it is rear facing. I think that applies to forward facing as well but not as sure on that.