I must admit, I didn't read all 5 pages of this thread...
I didn't realize the rules had changed about the rear-facing. With DD6, they only said rear-facing for a year. With DD4, I didn't do as much research, but I remember that they were starting to recommend extended rear-facing.
For my girls, we used an Evenflo Triumph, next I purchased the wrong seats and they had Evenflo Generations seats (not realizing how low the harness height was for these, when a kid reaches only 43 inches and then they have to move to booster mode) - my DD4 still uses these for everyday riding, my DD6 has a Britax Frontier and a Britax Parkway. I like the Frontier much better. When I purchased it, it had the heighest harness height on the market (things may have changed since then).
Anyway, I would highly recommend the Britax Frontier (which could be her last car seat), if they weren't recommending backwards seating for another 6 months for your DD. That said, I would recommend a convertible seat, but it's been so long since I used one... I must say that the Britax seats are much easier to install than the Evenflos, which allows you to have a secure fit even when you travel and are trying to install a car seat in a rental car while your family is waiting.
Be critical and don't just pay attention to the weight limits on seats. Your child is thin, so these will mean very little to you. Your child will likely exceed the height requirements long before she exceeds the weight limits. I seriously considered the Graco Nautilus for DD6, until I realized she would already need the top set of harnesses (when I went and tried it out at Babies R Us). You can go and try out the seats at Babies R Us without buying them there. I know it sounds like a horrible thing to do, but the seats can be $50 or more cheaper on
Amazon... Go try out the seats you asked about and any others that you are interested in, read the reviews on Amazon as well.
Remember the best car seat is one that fits your child, fits your car and that you will use correctly every time!