It all depends on what kind of game you like - you might like one, the other, or even (gasp) both!
Ninja Gaiden II is hard, at least I think so. Even on easy difficulty! While some may see it as a button masher (and it certainly can be played that way), it actually has a pretty deep amount of moves. If you've ever played Soul Calibur II with the weapons, you'll understand what I mean. There are enough combos to memorize and master that you probably won't use them all by the time your first run through of the game is complete. There's a fair amount of replayability if you want to play through the game over and over (there are achievements for running through the entire game only using each weapon..ick), but I'm pretty sure that most people (other than the hardcore) are going to stop if they manage to get through the game once. If you're an achievement person, this may not be the game for you. Anyone with 1,000 in this game has some pretty ridiculous skills!
Grand Theft Auto IV..well, it is what it is. If you've never experienced the GTA world before, it's a must-buy. All I've done with it so far is the first mission - the rest of the time is just sandboxing! It's such a time waster, like all those casual games from PopCap. I've played for a few hours, and those hours were all spent stealing cars, upping my wanted level, the usual stuff. I don't know why that's so fun, but I think it is. There's obviously a lot to explore and a lot of replayability..not to mention downloadable content for the 360 version, for those that hang on to the game that long. Who knows what (if anything) the PS3 version will get?
I'd say that GTA will probably give you your money's worth and then some, while NG is for the fans/hardcore/want a challenge crowd. It definitely won't be everybody's thing.