I played the original years ago and remember enjoying it. I've never played Rebirth, but I hear they're almost the same as terms of gameplay. I think one difference is that in Rebirth you get secondary weapons that you can use, along side the whip.
I'd say the original GameBoy version would be worth a buy if you're looking for a good Castlevania game, but this is coming from a big Castlevania fan. :p