I have recently found some information at www.carseatsite.com
You might also try www.consumersearch.com
Under the heading "Family" there is information on car seats. Mostly they tell you different websites that review car seats. Some are parent reviews, some are websites that rate the safety of the car seat.
For me, it was worth the $5.95 for a monthly membership to consumer reports just to get their ratings information.
From a personal perspective, I can tell you that I have been very happy with our Britax car seats. They are expensive, but they are worth it. They always score well in crash tests. And they score well in European crash tests, which are more stringent than the tests in the US. We have two Britax Marathon carseats. I moved my girls into them at 9-10 months when they outgrew the infant seat. My oldest is now 4 1/2 and she will probably be in this seat for at least another year because the carseat is for kids up to 65 pounds or 48 inches. If I were going to buy a new seat today I'd buy the Britax that has "wings" on the head (I think it's the decathalon) because the extra support around the head is safer.
Good luck finding a seat for your daughter.