Re: Question about different Britax seat?
The BV70 has bigger head "wings" than the Marathon. That's the only difference.
The CS refers to a "click & safe" feature that has a click indicator to help the caregiver determine when the harness is tight enough. It doesn't always work correctly, and ideally someone should know how tightly to pull the harness without it anyway.
The Advocate has exterior side impact "cushions" on the side, but otherwise is identical to the boulevard 70cs.