Castlevania is good, but Metroid is my favorite franchise. There's zero competition with anything against Metroid for me. :/
Metroid has better installments and adapted to 3d first person very well with it's prime series. Fusion has to be the best adventure/story But Symphony of the Night appeals to me much more....sadly that's where castlevainia ended for me. It's Castlevainia for me
Tough choice. If you mean the series, I'd have to choose Metroid - I bought a SNES for Super Metroid (and Animaniacs, but that's another story...) and I still think that Metroid Prime Hunters ruined things for other FPS's on the DS since it set the bar so high (and a game that will forever be memory-linked to my daughter's birth because I played it non-stop for a few days while Mrs. Drelbs was recovering and daughter was in NICU...) Now if you literally mean the original NES Metriod vs Castlevania, then it's a more difficult decision... but I think I'd still have to go with Metroid.