I've had both. The Scenera is currently our back-up seat in my dad's vehicle. The ComfortSport is GONE, thank goodness.
My tiny little not-even-on-the-charts son outgrew it rear facing at around 18 months. He outgrew it forward facing a few months prior to that, but I can't remember when since he never actually got a chance to use it forward facing. He was barely 20 pounds when he got too tall for it. Admittedly he does have a long torso, but it's rare a child will make it to 3 or 4 in this seat, and that's far far too young for a booster. It has really low top slots and they cannot be used rear facing. The top slots are for forward facing only. It was nearly impossible to reach the harness adjuster when rear facing. It ended up crammed against the back of the seat and I had to really struggle to get my fingers down in there to adjust it. It was a pain to install in my vehicle, but I have installed it in other cars and it was much easier there. Overall though, I just really didn't like this seat. The padding wasn't worth it.
The Scenera, however, has been great. My son has never complained about the lack of padding, even on 2 hour car trips. He just didn't seem to mind. He still fits in it rear facing, at 2 and 1/2. It has a fantastic 35 pound rear facing limit which I love. It's fairly easy to adjust the harness... at least I don't have near the problems I had with the ComfortSport.
In the end, you get far far more for your money with the Scenera than you would with the ComfortSport.