Both the Graco 4Ever and the Radian are very large seats, so I'm not sure one would work better on the plane (RF) than the other. The 4Ever is *relatively* compact for RF, so that might work better. Maybe (see below).
Because you have the GoGoBaby device, and because the Graco doesn't have a separate boot for RF, that
might work better, but the 4Ever is so new, that I'm not sure all that many people have used it on a plane (yet), so I don't know how well it fits.
One of the reviews
here, on the Graco website, says the 4Ever is a pain to use on the plane, and that it was also not easy to cart through the airport.
Could you consider purchasing a "travel seat" that might be able to later be used as a "back-up" seat? Perhaps a Guide 65, Scenera, or other lightweight, but fairly compact, convertible would work? Here are some recommended models for air travel:
http://carseatblog.com/25408/recommended-carseats-for-airplane-travel/ (this list is pushing a year old, but is still a good one).