The Graco Nautilus is a fabulous seat! I love that it harnesses to 65lbs (so long as they fit it by height--shoulders must be BELOW the harness slots), boosters to 100, even becomes a backless booster. The backless booster part has a NINE year expiration. That's practically unheard of in the carseat world. Even D's special needs seat doesn't have that.
I love the Radian too. We had one for D before we bought the Roosevelt that he uses now. Fab seat, nice and slim, low profile...just fab.
Shannon, does Landen have a shorter torso? If so, the XT is the best bet. If he's got a longer torso, the 65 is a better bet since he'll outgrow the XT by height before he hits 80lbs since both seats have the same top slot height for the harness. Devin was in the R65. He'd already outgrown it by height when we bought the Roosevelt. That said, Devin is very tall for his age and has the torso of the jolly green giant. JP, while taller than D, still fits the R65 (by height only) because he's all leg and no torso.