I, too, like Britax, but because money is tight right now, we just chose the new Graco Nautilus. My older son had a Marathon, which the baby now uses in one of our cars (rear facing), but he had to stop using it at 45 lbs (even though it says 65) because the straps wouldn't fit around his winter coat. We got the Nautilus for our second car. It's the last car seat we'll ever have to buy for him and so far seems to be great. At first I was concerned with having him front facing already, since he still seems like a baby...my older son was 8 pounds heavier at this age and walking, etc to make him seem older, but I know the baby's old enough and heavy enough.
I must say that at only 30 pounds, no, I would not put her in a booster seat using the seat belt. We just did it so early with my 5 year old (he was 3 1/2) because he really is a big boy, off the charts for both height and weight, and we couldn't afford another big seat.