Haven't ever used a Radian of any kind, nor a Boulevard, though we have two Marathons (in different cars) and a Frontier, and I like Britax.
But... the TrueFit is just SO much more seat for the money. You'll get more RF out of it for some kids, and more FF out of it for ALL kids. By a *lot*. My DS1, nearly 5 years old, could still use a TrueFit, but outgrew his Marathon months ago. It's the first convertible I've seen that I can truly see going from newborn to booster for one child.
The detachable headrest is a nice feature when you use it for an infant, too (you can leave it off until you reach 22 lbs., or until the top of baby's head is within 1" of the shell without it, whichever comes first). And it's my first seat I don't have to uninstall to adjust the strap height, so that's got me giddy. ;-)