Mercury vs Mars
Mercury is a lot denser than Mars, which some have suggested is due to abrasion of an originally Mars-like crust/upper mantle. The main difference is the lack of atmosphere on Mercury - I think that is the main driver of their differences. So much of Mars is all about erosion and sedimentation, none of which ever happened on the present Mercury. A cooler Mercury might've retained an atmosphere and perhaps they would've been sister worlds, but don't let appearances fool you - each is unique.