Our infant seat for Pickle (our nickname for A) was a Chicco Keyfit. This was a wonderful infant seat - got a really good close fit with the straps, we purchased an extra base that we could put in the other car, and so on. We were able to use it until she was a year, at which time she became a little annoyed with reclining in an infant seat, and it was too hard to carry her in it all of the time.
I did a lot of research, and heard nothing but great things about Britax. The top of the line seat at the time was the Britax Advocate 70 CS (LINK):
At the time, it was about $340. Since we had to get one for each car due to our work schedule, this was a pretty hefty purchase. (We had purchased one ahead of time when she was about six months as her first Christmas gift, because, hey, she was 6 months, what did she care?)
We really did like this seat - it was pretty great. The sides are incredibly thick, and great for side impact. The LATCH system makes it fairly easy to install, but we did have to do the pool noodle trick. The straps get a close fit (not quite as close as the Chicco, but still close) and are easy to tighten in a smooth motion with their "Click & Safe" technology.
The problem was once Pickle turned about 32 months or so. Britaxes are not made for children with tall torsos. Which is, unfortunately, how Pickle has been shaped since she was in the womb - they were worried that her legs were measuring a few weeks behind the rest of her... until they looked at J and I. We also have long torsos, short legs. Anyway. In theory, the Britax should be able to accommodate a toddler rear-facing until four years old. Unless your child is shaped like this:
(I didn't screw up her hair - it's both brown and blond right now.)
Here's a photo of her from when she was about 34 months - you'll notice that her head is almost exactly an inch below the top of the shell, which is all the taller you can be rearfacing in a Britax:
So, our options were either to forward-face her, which neither supporting statistics nor I were ready to do, or find a new seat in which she could continue to rear-face.
I did a ton more research, and found the Diono Radian RXT (LINK):
Not only is the rear-facing weight limit higher (45 lbs), but the shell moves up significantly, so it's great for tall-torsod children. Additionally, it works better with the new LATCH regulations (I'd recommend googling these, but HERE'S a brief description) - the LATCH will work for up to a 45 lb child rear-facing, rather than being the weight of the child plus the weight of the seat. This wasn't relevant to us, because we only have one child, but it's also super skinny, so you can fit three of them across in a mid-sized car, which we never could have done in the Britax.
Plus to Pickle: we could get it in purple. Also, she's super-comfortable in it. And, I don't know if I mentioned, we could get it in purple.
Clearly, it is also appropriate for pirate children.
It was also incredibly easy to install. J had always put the Britaxes in, but I put the Radian in all by myself and got a tight, correct fit. Unless you have a small infant unable to sit up unassisted, you will want an angle adjuster.
Anyway. Just because we're bound to breed tall-torsod children, any of our future children will likely just start in a Radian, or will maybe start in the Britax and switch to the Radian once Pickle outgrows it. (From what I understand, you usually can only use it up to the limits of the five-point harness forward-facing, which is 80 lbs. - most children grow too tall to use it up to the 120 lb. booster seat limit.)
I think both seats are awesome, but for children with a tall torso, I'd go with the Radian.