Now, I may need to preface this with saying that I do not own a PS3, but from what I've seen of the two... (I have played them both at my friends house, though)
Personally, I would shell out for Castlevania. The previous 3d Castlevania was absolute bollocks, but I don't know... Lords of Shadow has a much nicer design, I think, than Dante's Inferno. It's much sunnier, and brighter, and less brown and dreary, and I think the plot is a lot better. Though, the combat is a lot more difficult.
On the other hand, Dante's Inferno was absolutely hilarious. Not because it was a comedy game, or anything, but because it was so easy to make fun of. Honestly, gather round a couple of friends, and give it a go, and you'll die so many times because you can't stop laughing at how ridiculous half the game is.
Oh, and Satan is the greatest villain in all of video game history. Just saying.