Age and weight of child? RF in car? No personal experience with the Diono but I have a Guide, and have read about the Radian. I've read that the Radian will not fit RF on the plane, so you will have to remove the RF boot, and keep it in your luggage and then install it after your flight, which depending on how long your flight is, how tired you will be and how difficult you find the install might not be the best idea. I just removed our Guide from my car because it has a tendency to over rotate when installed if I don't use pool noodles and my dog ate the pool noodle a while ago...
Personally if it were me, I'd probably use the Guide or spring for the Scenera Next, but if you don't mind the Radian install etc and are sure it will fit on your car on the other end of your flight, it might be nice to have a light weight seat if just for lifting it into the car and installing it.