N.P.
I have had both the Britax roundabout and currently have the Radian. I loved the Britax (no twist straps, safe as safe can be, easy to install in any car). I also love the Radian for the same reasons. Changing it from rear facing to forward facing is easy, it is very slim so that helps, it is comfortable and soft for the kids and they like it. The only negative I had about the Britax was my daughters sweat tons in it, like it was a little heater. I haven't had that happen with the Radian. A negative of the Radian is that it doesn't have as high of side wings (I have the 65, not the 80) so they dont' have as much support when they sleep in it, but the infant who used it here at my daycare still fell asleep in it easily and comfortably.
The radian will be tall though, it may not work if you like your front seat far back or there isn't much room between the front and back. ((if you are in the plainfield area I can let you see my radian and you can try it in your vehicle))
I have a um, chubby, kid that was in the Radian and did just fine with it. He was not uncomfortable at all.