i'm shocked at how many people check their car seats! Ridiculous! Do you know that those car seats suffer the same damage being checked onto a flight as they do in an accident? The baggage handlers show no regards for it being a car seat, they throw it right into general baggage area, throw other bags on top of it, and it shifts around during flight. Imagine throwing a 50lb weight onto your car seat.... thats what happens when its checked, only its multiple 50lb weights (bags) thrown onto it, let alone the shifting during flight. Just because you can't SEE the damage doesn't mean its not there. Checking a car seat is never wise, it will suffer damage and guess what, that damage will be what breaks first in a crash, with your precious child in the seat who is DEPENDING on the seat 100% to save his/her life.
BRING the seat on the flight and strap her in. We've traveled with our Marathon many many times and never had a problem. Traveling with kids is lots of work anyway, might as well make your flight an easy one by having your child in her familiar seat. I think renting the CARES was waste... you have to have your car seat at your destination anyway, so bring it on the flight, strap her in, and save your car seat the damage it WILL sustain if you check it. Why check a $300 car seat only to get it damaged?
A duffel bag, or those travel bags for car seats, they won't PROTECT the seat from damage! Sure, damage to the cover or harness maybe, but it won't do a darn thing to protect the plastic, the integral part of the car seat, when its thrown around by careless baggage handlers and shifted around with heavy bags during flight.
Edited to add- When you gate check your car seat it gets put in with general baggage. it doesn't go to some special padded location where it will not get damaged, it just gets tossed in with the general baggage just like if you checked it with your baggage, but its brought out first and put on the tarmac for you to grab on your way off the plane.