I think in the end, being a Zelda fan, I'm more for the variety of weapons/tools. However, the problem with the both of them, IMO anyway, is that very few games do a good job of keeping you on the right general path.
The thing I hate about Metroid is that you can spend large amounts of time just wandering around looking for something that will trigger the next part of the game. I have just enough programming experience to really appreciate the amount of planning it takes to create a game like Metroid where you're constantly moving back and forth through areas, and needing to keep enemies balanced, not to mention set things up so that you can't access particular areas until you have this or that upgrade. In one of the Metroid Prime games, there was an area where it's a big geothermal plant, and waaay up in the ceiling there's a beam weapon upgrade... But you can't get to it until you have the magnetic ball upgrade, the little bombs for the ball mode (the name of which both escape me now).
I want a game that at least keeps me pointed in the right direction, not wandering around aimlessly until I happen to stumble across something important. If you have that, I don't care if it's Metroid or Castlevania style. In the end, a good game is a good game, and whichever way they go, the developers will have really thought it out and executed it well.