They've got different gameplay conventions, so using both is still kind of unspecific. In Metroid, the character has a gun, so the action involves shooting. Castlevania (at least ones I played) involved close-combat weapons. Metroid is about finding items and searching/scanning for breakable blocks and secrets; Castlevania has more straight forward level design, and the exploration aspect is about trying seemingly random things in seemingly random places.
So, which game is this more like? Metroid or Castlevania? If it's both, that's okay too... but I still don't like word morphs like "Metroidvania." It feels too informal for a professionally presented article, IMO.