The gamer bonus allows you to register the game with a special key (which is not the same as the M&B serial) on GamersGate. In return you can (re)download the game anytime you want to, should you CD get scratched. A nice idea really, but this is something every game should have and not be considered a bonus.
Anyway, in case this was not clear: The map and the poster are the same thing, its just the two different sides of it. Looks quite nice though,