I enjoy Magic Kingdom at Disneyworld more for one simple reason--people per sqare foot.  Disneyland is smaller (85 acres vs. 142 acres at Magic Kingdom) and almost as crowded (50,000 per day average attendance at Disneyland vs. 53,000 average per day at Magic Kingdom).  So, Magic Kingdom just feels less crowded because there is more space for the crowds, and that makes a huge difference for me.  But you also have to walk further between lands than you do at Disneyland.  It just depends on what you value more.  Also, at Magic Kingdom, during the summer, when it rains (as it does with relative regularity in Florida) in the afternoon, the park sometimes clears out...which is awesome.