I stole this off a car seat forum. I think it breaks down the differences of the "Big 3" pretty well:
What are the differences between the Britax Marathon, Boulevard, and Decathlon?
Marathon- The most basic model of the three. It harnesses to 65 lbs, rf 5 to 33 lbs. It has a rf tether, ff and rf lockoffs. Has the lever style harness adjuster, 4 harness heights, and the crotch strap does not adjust. It is very padded and comfy just like all the britax seats, and has about a million cover choices. Will last till the child is ready for a booster most likely. Good for children that need to rf still (under 33 lbs).
Decathlon- Harnesses to 65 lbs rf 5 to 33 lbs. Has rf tether, ff and rf lockoffs. The ff lockoffs may damage seatbelts. It has a push button style harness adjuster, 4 harness heights (rumor is they may be a little taller than the ma/bv), and a three position crotch strap that gives 1 more inch of room as compared to the marathon and boulevard. It has infant inserts so is great for a small baby. It also has a removeable toddler head pillow. It has more limited cover choices than a marathon. Will last till child is most likely ready for a booster. Good for very small babies because of the infant insert but a ma/bv may be a better choice for an older child that won't need the infant insert.
Boulevard- Harness to 65 lbs, rf 5 to 33 lbs. Has rf tether, ff and rf lockoffs. It has true side impact protection (the "wings"). Has a lever style harness adjuster, and instead of having to remove the seat from the car to adjust the harness height you just turn a knob on the side of the seat, and it does not have an adjustable crotch strap. It has a removeable infant pillow so is also a good choice for a very small baby. Will last till the child is most likely ready for a booster. Good seat for any child. Especially one that will have to be on the side of a vehicle without side curtain airbags.
The Roundabout is a good seat but for just a few dollars more I'd get the Marathon because it will last you literally years longer, especially if your genes tend to be on the taller side!
hth