I've only played God of War and God of War 2, but I have to say, GoW2 is such a better game that there's really no contest between the two. While I love GoW1's setting and story, it's severely flawed:
- far too short
- incredibly uneven difficulty, particularly the rotating blade column you all know and hate, and the entire final boss sequence
- riddled with game-breaking glitches (I experienced one in the Temple of Pandora that forced me to lose several hours' progress)
- no real reason to replay the game. Your skills and abilities don't carry over to subsequent playthroughs.
GoW2 is better than the original in almost every way. Its only real drawback is that it's a sequel, so for some players it can never equal the impact of the first game. Let's make a brief catalog of GoW's many advantages:
- longer than the original, but not too long
- extremely varied and well paced, in terms of boss and enemy design as well as the environments
- has useful alt-weapons along with an instant weapon switching system
- has an expanded combat system that allows for something resembling combo-oriented high level play (Nothing on the Devil May Cry series' level, but still very respectable for an action/adventure game.)
- loads of bonus content that extends replayability and enriches the base game, including...
- a Bonus Play feature that lets you take a (nearly) fully equipped Kratos through the full campaign
- a customizable arena mode
- better bonus costumes that change Kratos' attributes
GoW2 is an extraordinary sequel. I'm confident that if anyone were to revisit GoW1 and GoW2 right now, the sequel's considerable design improvements would be obvious.