I think Castlevania is a lot more accessible. There's more action, more plot, more easter eggs, more anime influence, I believe it has more sequels in the series and makes it easier for people to find the one they enjoy. They're all slightly different.

My experience with Metroid is that the few I've played were almost too similar. Very empty. Very by-the-books on the exploration and attaining items. Lots of backtracking as well. It didn't seem like there was as much meat. Though for those who appreciate it, it's probably a "less is more" mentality. Which I respect, but it's sadly kinda lost on me. And I'd wager it's lost on most people who prefer Castlevania. Developers included. But I think the core of the Metroidvania'ness is just that there's a map to explore, and ways are blocked, and you need items and upgrades to advance. Sprinkle in some canon fodder and a few boss fights and they'll call anything a Metroidvania