I just bought a Radian today for my almost 5year old. He has an extremely long torso and has just about outgrown his Marathon. But he is only 35 lbs so I needed more time for him to be harnessed. The Radian fit the bill. It is definitely a seat for tall skinny kids.

That being said, since I own both... I would absolutely get the Marathon. It is much cushier, has much nicer harness straps, and is easier to install. I've put Aiden in the Radian three times today installing it in various vehicles and already I am missing my Marathon straps. I am going to need to be careful with the Radian straps because they are no where near as thick as the Marathon's and could twist easily. The Radian is a nice seat but it doesn't come close to my MA, IMHO.

I also only bought the MA for Aiden a year ago, but would do it again in a heartbeat. I love my MA. The good news is that now his younger brother has two MA, so I only have to move the Radian between vehicles now.

HTH, Jen in Canada with
Aiden 4.75 yrs in a Radian
Mikkel 21.5 mo in a MA