I've flown with both!
I have to admit that I used both ffing because we also had the Sit & Stroll, a very convenient product but outgrown about 6 months earlier due to very low shoulder straps (I think 14"). It's a combined car seat/stroller and as a Jack Of All Trades, does neither that well. For the price; around $200, it was great as a baby gift but I can't recommend it if you don't do a LOT of air travel and/or are having more babies, both of which applied to us. It fit well into airplane seats, could roll down the aisles and it was great to not have to take the stroller as well. It is, IMHO, the "best" seat for airplane travel but not worth the money unless you really are going to use it a lot.
The Radian is super easy to take, only slightly in second place. Instead of the expensive GogoKidz cart, I just use a regular metal luggage cart and secure it with an extra bungee cord. My child could ride in it just as well. The Radian can also be wheeled to the seat, it's narrow enough while the Scenera has to be carried (although it isn't very wide).
I think taking a cut up pool noodle will mean you can rf with the Scenera on board no problem, or just a towel or blanket. The Radian is tall and hard to rf on the plane as has been mentioned.
My dd was in the Scenera till age 3 1/2. That's not as long as the Radian, which she's still using at almost 6 years old (she's slightly above average size). But 3 1/2 years is still a good run for any 40lbs limit seat. You might have another child by then!
While I agree with the pp over the Rider Safe vest, it can't be used on airplanes, so I thought I would mention that. At three years of age, she would be allowed to use only the lap belt, although you may or may not be comfortable with her flying like that. A CARES harness wouldn't be worth it since it only goes to 44lbs.http://www.kidsflysafe.com/
Watch out for that because I understand that they are working on a model that will go to a higher weight. This might be the case three years from now when she outgrows the Scenera!