I already have a Boulevard (you can see a pic of my 20 month old in it here
And was debating on whether to get another Boulevard or a Radian for number two. The main reason I chose the Radian is because it fits newborns much better than Britax convertible seats.
-The Boulevard has built-in lock-offs for when you have to install in vehicles without locking belts, or if you get too much tilt when installing your seat. The Radian, in this situation, would need to use a locking clip (total PITA, lol)
-The Boulevard has infinitely adjustable harness height--just a knob that can roll them up and down, even with your child IN the seat... however, the Radian's top harness slot is 1" higher than in the Boulevard, and since most kids outgrow seats by height before weight (once they are forward-facing, anyway) this can be a big perk. The Radian is also the only seat on the market that allows you to harness your child AFTER they are above the top shoulder strap slots, although this is very highly debated and most don't do it (generally, for forward facing, straps must be at or ABOVE shoulders).
-The Boulevard is insanely easy to install everywhere I've put it, ever, period (and we've test-driven about 18 vehicles lately, so it's gotten around!). I've done seat-belt and LATCH, both are a breeze. The Radian, I hear, CAN have install issues. However, the Radian is thinner width-wise than the Boulevard (although it DOES take up an obscene amt of room rear-facing, lol)
-The Boulevard is deep, the Radian isn't. Meaning, you'll get more air flow to the rear-facing child in the Radian, but you'll also lose more sippy cups and pacis over the sides.