We have a Britax Marathon 70 and I LOVE it. I practically drove myself nuts researching carseats and I'm so happy with my decision. It fits great in the back of my not-too-big car. We switched to a convertible around the same time as you. DD has always been off the charts in height and 90th percentile in weight.
We also had the Britax Marathon but we absolutely hated it. My son is also off the charts on height and in the 80th percentile for weight and the Britax was just a very poor fit for him. I also had some issues with the straps not getting tight enough that
were never properly resolved despite multiple calls to Britax and a
couple of trips to have techs check everything out. I registered for our Britax but didn't learn until later on that Britax was not generally recommended for taller children since the shells are shorter. (I guess I also didn't expect such a tall child either!) We used it for about a month and half from about 6-7.5 months before I decided to get something new. It just didn't work well for my son.
We have the Clek Foonf now and love it but I would not recommend it for a smaller car because, rear facing, it's gigantic, taking up a lot of forward space as well as height. It is also really not the definition of affordable, either but I liked that I could keep him rear facing longer and, hopefully, I'll get a little more time out of this seat since it does have high height and weight limits.