It's Convertible Time!

We love the britax marathon. After research and a fatality accident in one, I'm convinced of their safety ratings! Ive also heard good things about the sunshine seats people have mentioned here. Ive always found the best deals online at www.albeebaby.com or by buying new in box on eBay.

both of those seats are very good - the high height and weight limits ensure that your daughter will be able to use the harness until she's got the size and maturity for a belt positioning booster. I'd recommend going to a store where you can try both of them out (stores like BRU *should* allow you to test the floor model of the seat in your car to see how it fits) and see how they fit your DD and how easy they are for you to install and adjust.
other good seats to consider are the compass TrueFit, the Evenflo Triumph Advance, and Sunshine Kids Radian. http://www.car-seat.org is a great place to bring any car seat related questions - many folks there are certified techs with loads of experience installing car seats in a variety of cars

We have a britax and I love it. Ours goes up to 33lbs rear-facing and 65lbs forward facing. I bought it before my daughter was born so it's the only seat I've had to buy for her. When she out grows it she'll go into her brother's seat that goes 5 pt harness to 80lbs and he'll go into our britax high back booster that goes to 120lbs. Can you tell we love Britax!?

Remember she'll need to stay rear-facing until at least age 2 (or beyond) but 2 is the minimum recommendation. I just turned my daughter at 31 months because she was nearing the weight limit. But she is in the 95th percentile for height and she was perfectly comfy rear-facing.

We have two convertibles -- a Britax Boulevard and a Sunshine Kids Radian XT. I'm indifferent on the Britax. I think the Marathon would have been fine and we didn't need the additional features. Looking back, I would've bought two Radian XTs. I love the seat because of the higher rear-facing limits and low profile. Good luck!

EDIT: I wanted to add that after lots of research and reading reviews, I went with the Radian because of the higher rear-facing limits and generally positive reviews. I had purchased the Britax several months earlier mainly based on the name. At 18 months my DD is still RF and I hope to keep her that way as long as possible. The Blvd has a higher profile, and it's getting harder to get her in there with the seat rear-facing (the Radian on the other hand sits nice and low). I think the latest carseats have higher RF limits. That might be something else for you to consider.

Finally, I bought both seats through diapers.com. They have a great return policy, next day shipping, and online price-matching. If you want to see the seats in person, look for a Buy Buy Baby near you.... they tend to have a much better selection than Babies R Us. Good luck!

We LOVE our Britax Marathon. I have one in my Saturn SL1 and we have one in a minivan. Works fine in either car.

We love our Evenflo Momentum 65 car seat. I've had a few Evenflo car seats (triumph, momentum, etc...) and love them all. I also had a very small car (Toyota Yaris) and that was the only one I could fit that would last my son for a while!

We have the Graco MyRide 65 and I've been very happy with it. My son is 21 months now and he's been in it since he was 8 months.

I love the britax Boulevard, I got hit by a drunk driver and my car was crushed, my daughter was on the side that was hit (long story shes ussually in the middle) but the car seat and her were the only things intact, all other passengers had injuries, she didn't have a sratch, I replaced it with another. Sure its big, but it's safe, and will last, i got the britax boulevard 70 so it will last a while this time, this car seat is still one of the safest on consumer reports every year!!! Britax Boulevard def. is my vote! You can but cup holders to attach , thats theonly thing I didn't like about it

We have an Evenflo Symphony and love it! My husband has a 4-door sedan and it works really well in it. They recently "upped" the weight limit, so the one in my husband's car will last us well into elementary school as a booster. It's much less expensive than the Britax with very comparable safety ratings.

I love my Britax Marathon. I remember doing lots of research before I bought mine and was pleased with the safety ratings I read, as well as other moms' happy responses. Latch system is super easy, seems comfy for DD and no problems with the straps. I don't know much about the Boulevard 70. I'm about to move DD#2 in the Marathon, and bought the Britax Frontier 85 (car seat/booster combo), for DD#1. Good luck!

*also, I kept DD#1 rear facing until 21 months...try to keep them rear facing as long as possible!!!