I voted Castlevania, although I have great love for both series.
When I was 6 years old, if memory serves, my brother and I would walk a great ways to this bowling alley that had the original Castlevania in it's arcade. We'd play it (I'd mostly watch, being so young)
When we got our NES the year after, it was one of the first titles I wanted.
I was never very good at it, didn't ever get that far into it back then... but I loved it, loved the music.
We also played Metroid at that time, and it was fantastic.
I loved Castlevania 2 (though it was a bit odd) and 3. The next time I touched the series was Castlevania 64, which I liked more than most.
But there's no denying that the Castlevania series has had a rougher transition to 3D than Metroid. Metroid Prime series was fantastic.
Most people I guess don't pay that much attention to the 3D Castlevanias, and they prefer the current 2D stuff.
Personally, I don't care for them. I tried Symphony of the Night, and it seemed kind of cool... thing is though, I hate all that RPG shit they infuse into it, and it's only gotten worse on the more recent ones I've tried, for Nintendo DS and such. Too anime'd up also. I don't care for that excessively anime art style, the shemale main characters... a billion stats and menus to fiddle with. I liked the simplicity of the original entries in the series.
Metroid 1 was fantastic. Great music in that too.
If I was comparing Metroid Prime to Castlevanias these days, I'd give it to Metroid easy.
But original game in each series, I give the slight edge to Castlevania.