Over the last three seasons, Lampard has outperformed Gerrard (I truly can't see how anybody can argue with this in the first two, although Gerrard has been a little better this season). However, I don't think anybody can dispute that Gerrard on his day is better than Lampard could ever be.
Gerrard's problem is quite simple - if things aren't going his way he gets easily frustrated, and his response more often than not is to start playing 'hollywood' passes that succeed once in a blue moon. He's got all the potential in the world, but until he learns to channel his drive and determination he'll forever be someone who could've been a great but got held back by his temper. The strange thing is that it seems like a 50/50 thing with him, he'll get frustrated in one game and get petulant and start making stupid long passes, then in the very next game he'll get frustrated again and will just take the game by the scruff of the neck and drive his team forward.
Personally, I think Gerrard is tailor made to join one of the big Italian clubs where they emphasis tactical decision-making; if he can improve in how he uses the ball he would be almost the complete player.