The Britax Marathon and the Britax Boulevard are both convertible child safety seats, meaning that they can be installed in either direction: rear-facing for infants and forward-facing for older children. The two models share many features, but there are a few key differences. The Boulevard is the "higher-end" model of the two.
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The models have identical overall dimensions: 18.7 inches wide, 26.9 inches high and 24.4 inches deep. However, the Boulevard allows more shoulder room for the child: 13.5 inches wide at the shoulders, compared with 11.75 for the Marathon. The Marathon is lighter, at 17 lbs., while the Boulevard is about 19.5 lbs. Both have the same specifications for child's age and weight. Children must ride rear-facing until they are at least 1 year old and weigh at least 20 pounds. Both models can accommodate a rear-facing child between 5 lbs. and 35 lbs. and a forward-facing child up to 65 lbs. Slightly larger versions of each model are available that can accommodate rear-facing children up to 40 lbs. and front-facing children up to 70 lbs.
The basic safety features of the two models are the same: an expanded polystyrene foam liner and a deforming-plastic base that absorb and diffuse the energy from a crash, plus a layer of "High Density Comfort Foam" for cushioning. Both models also feature Britax's Versa-Tether system, which anchors the top of the seat at two points, rather than one, to reduce the motion of the seat during a collision or sudden stop. For ease of use, they both have push-button-release connectors for the LATCH anchors installed in most cars, as opposed to clip-style connectors, which require you to wedge your hand into the seat cushions to undo them. And both have harness holders, to keep the belts out of the way while you put your child in the seat.
While both seats aim to keep the child contained in the seat and prevent pieces of the car from intruding into the seat in the event of a crash, the Boulevard is also designed to keep the head, neck and spine aligned during a collision to further reduce the chances of injury. The Boulevard also has heavier padding behind and to the side of the head for both comfort and protection, as well as a "body pillow" to ensure a more snug fit for infants. The Boulevard also has a "no-rethread" system that lets you adjust the height of the harness above the shoulders without having to disassemble the harness and thread it through a new set of holes.
Both models have passed the the National Highway Traffic Safety Administration's crash tests. NHTSA also rates car seats on "ease of use." It gives the Marathon three out of five stars for both forward-facing and rear-facing installation. The agency gives the Boulevard three stars for forward-facing installation, two stars for rear-facing.
The Boulevard is a little more expensive than the Marathon. For example, as of mid-2010, Britax suggested a retail price of about $280 for the Marathon and $310 for the Boulevard.