I went to Buybuy Baby to look at car seats. This is my first time buying a convertible car seat. The sales person told me that the car seats are all basically the same when it comes to accommodating children of different height, although that's not what I've been reading. My daughter's height is in the 95th%. We currently use the Britax B Agile travel system. I like it other than the fact that we cannot keep the infant car seat in the car in the sun because it burns up (I live in South Florida). After talking with the sales person, I was convinced that the Marathon was a good choice and liked the idea of getting another Britax. My father installed it, with my help, and it was a PITA. I've taken LO out twice in it and am very disappointed. I don't know if this is normal for all car seats, but I find it difficult to get her in and out of it in my CRV. I feel like if she were any taller, I'd hit her head taking her out. Plus her feet already touch the seat in front of her. Not to mention it has no head support on the sides. I'm planning to return it and am putting the infant car seat back in for now. In the mean time, I'm trying to search for a better one and finding it quite stressful! I can't seem to find one good enough for our needs. I was looking at the Chicco Nexfit, but I'm reading complaints that the straps are difficult to get tight enough. I need a seat that's comfortable for a tall child, easy to use, comfortable and doesn't burn up in the sun. Any suggestions??

Re: DIsappointed with Britax Marathon and have no idea what to go with
You may want to look also at Recaro Seats and the Evenflo Symphony.
Scratch the idea of a Safety 1st Advanced Air with a 95% little one, and Recaro, and the Symphony. None are really long lasting for tall little ones. I'd go for the Graco MySize/Headwise/Size4Me, lower profile than Britax, and the tallest shell on the market. It also fits well in smaller spaces, and is fairly well priced if you can find the discount codes (I hear Kohls has good deals). The Nextfit has really high sides so that may not solve that issue for you.
I would also look at the Diono Radian, it has a nice low profile and also works well in small cars if needed (although I would recommend ordering an angle adjuster as well, it might me needed to get a more upright seating position. Both the Radian and the Graco seat will get you to at least 3 year old rfing, which is pretty decent for a tall lo.
Emily 8.8.08
Madeline 1.2.11
William 8.5.12
Emily 8.8.08
Madeline 1.2.11
William 8.5.12
We have a Britax Marathon and love it! DD is also very tall; she always ranks in the high 90's in height and was at one point in the 100th percentile for height. We are still RFing at 21 months. It can be a pain getting her up and in the seat but once in she is comfortable and we don't have a head support issue.
With that said, my ILs have the Graco MyRide 65 and it does have a much taller shell. Also her legs aren't as cramped and she can almost have them straight. However, her head does droop and I hate the harness fit.
Married Bio * BFP Charts
-top of shell, op, just to clarify is not the headrest. I've seen the seat outgrown and being used as they were measuring from the headrest and not the shell.
Just for a visual, my girls are all over the 95% and none would have made it to two rf in Britax seats (I didn't own them, we tried them out, I am a sucker for cute fabric). The actual limits on Britax seats are very overstated. It's 24" to the top of the shell, so seated your dd can't be taller than 23" to the top of her head. A MySize is 27.5", so seated height of 26.5", which is a huge difference. Most kids get 1" torso height growth per year, but that's not typical at this end of the growth curve, look for at least 1.5".
I also in S in a MyRide as a back up seat at 27 months, less than one inch of head space. If I could have a MySize, I would in a heartbeat, but we don't have them here. The harness issues that the pp mentioned are far and few between now and been addressed, and Graco has handled the customer service on this quite well.