I think 2 is better. But they tell a compelling story about a family. We think we are getting a glimpse into a world we don't know about.
The story (the rise and fall of a powerful man) is archetypal. And while at the end Michael has defeated all his enemies, he is alone and has no one you share his triumph with. This is ac story as old as time itself.
The story examineed Michael's journey as he ignored the family mantra, "It's not personal. It's business." It was always personal for Michael. From the time that McClosky broke his jaw and Sollozzo tried to kill his father, Michael took it personal. In the end Michael could never let anything go and it cost him everyone that he loved. It's a tragedy and sometimes we like to watch tragedies.
The casting, acting and direction is superb. Theb fact that the first movie was shot on a shoestring budget and turned out so well is pretty amazing. The pacing, despite both movies being very long is excellent. And the music is a close to perfect as there is.