Every car seat is SAFE when correctly used. The reason that people consider Britax seats to be SAFEr is because they are easier to use. If it's easier to use correctly, its more likely to be used correctly. The differences are as follows.
The marathon and boulevard only have one crotch setting. The decathlon has 3.
The marathon and boulevard both have metal bars on the side that the LATCH connectors attach to. The decathlon does not. It just has the strap threaded though the belt path on the back.
The adjusters to tighten the marathon and boulevard are a ratchet type (pull release, pull release) to tighten. You lift a leaver to loosen. The decathlon is smooth. It's a push button release.
The marathon doesn't have any extra padding inserts. The decathlon has a pillow and an infant insert. The boulevard has the infant insert as well.
As far as the straps go, the marathon and decathlon have the 4 height strap settings. The boulevard has in infinite (you never have to re thread it) height option.
The boulevard also has the SIP wings.
They kind of go from your basic marathon to the decathlon with extra inserts, to the boulevard which has it all. They are all on the same (roughly) shell though.
I grabbed these off the internet to kinda show you.
MARATHON
DECATHLON
BOULEVARD