What size shirt does he wear? The Radians are outgrown by most kids by height (not weight) while I cannot imagine the child that will outgrow the Gracos by height instead of weight (they'd have to wear a size 7/8 shirt, but weigh less than 40 pounds).
Based on your son's current weight, he is predicted to hit 40 pounds (rear-facing limit of the Graco) after 5 years of age. Based on his height he would fit in either of the seats until after four rear-facing unless he has a crazy long torso.
I'd pick between those based on which seat you prefer. For forward-facing, the Radian's booster capacity will be basically unusable for your son, BUT it does allow the shoulders to go over the top slots (as long as the ears are still contained) so it could last longer. Either should get him to a safe booster age, though (when you can get a dedicated booster).
If you have a Babies R Us nearby you can try out the Graco in your car to see how it fits. Some people have had harness issues with them (not tightening enough) so be aware of that (Graco is being good about refunds/ exchanges). If your budget is open, you could also look at the Chicco Nextfit which has a 40 pound rear-facing limit, is between the Radians and Gracos in height and is a much plusher seat (if that appeals to you).