K., we had the same dilema when the twins hit 20lbs around 6 months old! I did a lot of research and comaprison shopping and we bought the Britax car seats ($289). I know it sound like a lot but it goes up to 100 lbs.
The Down Side For Us..
They are a bit bulky, so it was a squeeze(for me)for a little while in our sedan, so I had to move my seat up so the seats would fit. They are also a bit wider than other car seats so my 9 year old son (who sits between them) isn't as compfortable as he was use to with the infant car seats.
The Up Side...
If we had less kids, we would not have had the challanges mentioned above. The twins LOVE their seats, because it allows them ot look out the window, they are rear and forward facing up to 100 lbs (so we don't have to buy more car seats for a long time!) They feel safe and they have room togrom, they are also very easy to adjust as they grow.
Good Luck on your hunt!