Britax seats are awesome, super comfy and top safety ratings. Which model to choose though? Here's my research FWIW.

The Roundabout has been a longtime favorite, but the weight limit only goes from 5 lbs to 40 lbs before you need a booster seat, so unless you need a lower pricepoint upfront, a Marathon or Decathlon is probably a better long-term investment.

The Britax Marathon (approx $250) goes 5 - 65 lbs, so it lasts a lot longer without additional purchases. Britax just came out with the Decathlon, a step up (sort of?) from the Marathon, and is about $40 more.

Here are the differences:

Both the Marathon & Decathlon have the same frame, fabric grade, level of cushiness, weight parameters - essentially the same seat. The Decathlon comes with a detachable head pillow for behind the head (nothing too impressive IMO) and it also comes with a cushy insert that would give extra padding to surround an infant, so they don't seem "lost" in a giant seat. Seems like this would be most useful if you were going to use the seat from birth and not ever use an infant carrier-type seat. By the time baby has grown past a carrier, I don't think the insert on the big seat would be used for very long.

The only design differences are that the Decathlon's crotch strap where the harness buckles into, has 3 different slats (like the slats on the seat back where the straps come through and you can adjust as they grow.) So for infants you would run the crotch strap through the slot closest to them, and then move it out as they grow. Also, the harness loosens/tightens by a push button release on the front of the seat, rather than a little hidden finger-lever at the base of the crotch strap on the Marathon.

I've read that some people felt the Marathon's seat belt straps that come across the legs to the crotch were squishing their chubby babies' thighs a bit, so if you have a big/chubby baby, the Decathlon might be a better choice, given the adjustable crotch strap. However, I also read the only criticism of the Decathlon is that the pushbutton feature makes it a little more difficult to loosen/tighten straps - I assume this would be while the seat is rear-facing, probably because that area of the car seat fits so closely to the back of the regular vehicle seat and it might be awkward to get your hand in there to push the button.

Conclusion: I got the Marathon for my 7 mo old daughter (my son has a Marathon as well, purchased in 2003 - love it) because 1) she's almost too big to need the Decathlon's infant insert, 2) I didn't think the head-pillow was anything much, and 3) my son's almost 3 and has never had any problem with thigh-room in his Marathon -- my daughter will likely be normal-to-slender as well, so I don't anticipate a problem with the single-position crotch strap.

The seats are identical in all other ways, so I saved the $40 and got a Marathon.

Whichever model you choose, Britax rocks - 5 stars!

Also nice, you can buy different covers seperately, so go ahead and choose the girly floral fabrics - you can always buy a tan or denim boyish one later if needed for a subsequent child. :)