What is your favorite entry in both franchises?
Super Metroid and Castlevania: Symphony of the Night
What is your favorite of the two franchises and why?
Metroid.
The gameplay is geared more towards testing the player's dexterity, reflexes and mastery of the game's many intricate mechanics, especially if you want to go for 100%. I feel like some of this naturally going to be lost once you start designing these games with RPG elements in mind.
Finding as many items as possible in a 2D Metroid game requires you to solve some really vexing puzzles that the game creates using its own environment. Castlevania games have an element of this, but the emphasis on map completion rather than item collecting means that it's often more about pure exploration and less about puzzle solving or manipulating mechanics in clever ways to try and find secrets.
I also feel like Metroid nails the difficulty curve better than Castlevania games. Symphony of the Night for example is the best game in the series but has a horrible difficulty curve. It starts out as a moderate challenge, then becomes really easy once you level up a bit, then there's a *very* sudden and abrupt difficulty spike as soon as you get to the inverted castle, and then it just becomes an absolute joke again once you get the Alucard shield/crissaegrim. Metroid games by contrast have a much smoother, more linear difficulty spike even as you acquire new power ups, which is how it should be. And yes, I know SOTN becomes more of a challenge if you want to impose arbitrary conditions on yourself like "no game breaking items", but the fact that such game breaking items are there in the first place points to a lack of polish in design that you simply don't see in Metroid games. I mean, the Alucard shield isn't even hard to find and it practically makes you invincible for the rest of the game.
There's a post on this forum
here that compares Super Metroid and SOTN in greater detail and explains why Metroid is the better designed game.
Were they surpassed by other game/franchise in the same genre?
Never played a non-Metroid or Castlevania Metroidvania so can't really say.
Castlevania, the music on those games is much better than metroid's.
When removed from the context of the game, maybe (although even that's debatable), but I feel like the music in Metroid works within the game to create atmosphere more effectively than Castlevania's soundtracks do.
Fwiw, I think the classic Castlevania's like Super Castlevania IV, Bloodlines and (especially) Rondo of Blood have the best music in the series, but they're excluded from OP's discussion.