I really do enjoy both of them equally... There's a reason I enjoying playing Castlevania... it's not because it's centered around hunting Dracula, the most powerful and pimp of all vampires, or that you're playing as a manly man in a leather skirt using a whip, or that it constantly references Tolkien and a lot of religious legends, though these do help... It's because it's so ridiculously fun.
The Classicvanias offer the one thing I want in a video game: a very fun challenge. I haven't beaten a single one yet except Dracula X Chronicles, yet I keep going back and playing them... when I play a classic action game, I want to die a lot, I wish to game over constantly, just so that beating the game actually feels like an accomplishment.
Metroidvanias aren't as challenging, no, but the same extremely fun gameplay is still there. Yet they do something else for me: they add variety to the gameplay... And when you get extremely powerful leveling up, they do something else for me: they make me feel powerful. I enjoy working my butt off to get the most powerful weapon just to use it and feel very accomplished while using it. It pays off. Yeah, you have to grind in Metroidvanias, but to me, destroying undead never gets old. And if I feel I need a challenge out of the game, I can just play as the hidden character or an alternate mode, which in every Metroidvania I played, is always awesome and badass when you're playing it, but you also almost always die in about 5-8 hits, which it then starts to test your very skill at the game, which I enjoy so much.
I've also played Curse of Darkness, which I enjoyed it about as much and for the same reason I enjoy the Metroidvanias, so I won't go on much there.
The other 3Dvania I played was Lords of Shadow, and I couldn't find much fun with the game because the combat system wasn't that good IMO... It just wasn't as fun; My whip's power never got stronger, so it was challeging... but when I'm practically spending a minute or two just to kill one minor enemy, and then they swarm you, the challenge doesn't feel fun... That was really my main problem with that game, and for right now that's all I'll say about it...
Lord of Shadow aside, for every Castlevania game I play, if I'm not haven't fun while playing it, then I must be doing something wrong.