hero,
If you enjoy the NES Castlevanias, you'll love SC IV. It has all the classic bosses, the soundtrack is godly, has a fully-controllable whip, and the game even has cool Mode 7 effects! If you've never played the game, you will die a lot since it is a challenging platformer. I would say it has less replay value than SM but it's one of those play-in-one-sitting games. There's also a considerably harder mode once you beat the game. BTW, the game runs on passwords and there is no save system.
SM is an awesome game, too. The game is huge (lots of locations), boss fights are epic, and the powerups get better and better. However, progressing through the game may be frustrating at times if you cannot figure out what to do. You may even be stuck for days or weeks trying to figure out a problem. Just make sure not to use a game guide unless you are completely stump.
All in all, it comes down to gameplay preference. Both are great, eerie games and you can't go wrong with either. I would suggest you get both! Although my opinion is bias, I personally prefer SC IV b/c I grew up w/ that game, know almost every secret, and it's the best Castlevania I've ever played.
And if you've never played any of the Metroidvanias, you should definitely try one. Just don't play Harmony of Dissonance on GBA b/c it's not that great when compared to the others.