Ive used the CARES System twice. My daughter now uses the regular seat belt on planes..
Both times we used it we flew from NY to FL - so it was a little over 2 hour flight - but here is my story....
I had the CARES and my sister had a car seat for her son... The CARES weighed less then one pound and fit in her carry on bag. It was simple to get on and she was all set faster then my sister... The first thing my sister had to deal with is finding out if she had an approved seat for Air travel - not all car seats are approved but it should say it on the seat itself - she went and purchased a new car seat. Then she had to carry the seat throughout the terminal and thru the airplane. We placed it into the stroller (which you can check it at the gate) but it was still bulky and a real pain. Its not the easiest thing to carry.
I never had a problem using the CARES system - the only problem I could see you having when you little one decides its time to sleep. It is a harness but you can only lean back as far as the airplane seat will go. Unless you use one of those pillows to help with that. Your car seat my be able to recline somewhat.
On my most recent flight back from Hawaii - 10 hour flight - we were able to basically lay down (the plane was empty) - we were also able to walk thru the airplane during the flight. If you have a connecting flight it will be even more of a pain to remove the seat more than once. Not sure if you already booked your seats, but while I was on that plane - there were a few babies on the flight - they took seats that were right after the bathrooms not in the back of the plane but in the middle of the plane - they were able to attach these devices against the wall to put their babies in during the flight - it was like a bed for the babies - it was the coolest thing I have ever seen! You should look into that too!
Also - I never purchased the CARES system, I rented it from someone on ebay - cost me $15 - but saved me the sanity of having to drag the seat - we just checked it in - which was FREE.
Good luck in your decision.