When my baby was about 7 months old she had grown out of the infant seat and I started to shop for new car seat. I looked at Graco convertible seat, Eddie Bauer seat, many more.
The only seat that I liked was Britax. I like:

• the construction!!
• good fabric
• nice straps (the straps have little velcro to stick them on sides of seat after you take your baby out, velcro straps on side and then when it's time to go back in the seat the straps are not on your way)
• the shape of seat, the back is not too straight, it's nice comfy back
• easy to install

Roundabout is a little smaller model than Marathon, Marathon seems to have higher back. If you have a really big baby I guess the Marathon is better? My baby is very tall and she is comfortable in Roundabout.
This is my little review of the Britax, hopefully it was helpful.