The cosco scenera is a great lightweight travel seat. It has a reasonably tall shell (so will last to a decent age rear facing) and is very lightweight. It's a bare bones seat, but passes the same test as any other seat on the market.
I wouldn't want to travel with a radian because they're incredibly heavy (I have 2). Lugging that through the airport would be miserable.
Also an FYI, but the carseat should be used on board for two reasons. The first is obviously that it's safer for your child to be properly buckled in a moving vehicle (in fact, the FAA recommends this even though most people are unaware of it) and the second is that it's very unsafe for the seat to be checked as baggage. Seats often come out seriously damaged and it's in the form of microscopic cracks in the plastic. You wouldn't know there was damage until the seat fails in an accident. Not to mention that if the seat became lost baggage you would be stuck in the airport with no way to transport your child at the other end.