The second fills that out. You see Vito as a young man, beginning to realise his particular gifts and choosing the life he does. And simultaneously you see the fully developed Michael who no longer fights his role and now easily takes on where his father left off. I'm probably a little passionate about these films because I caught Godfather II on Foxtel recently and felt like standing and applauding at the end.
Well, with Michael I think you can see in the first one that he has always been jealous of Sonny, but has not been in a position to do anything about it...but then he takes his chance and gets what he wanted, his father's respect and his role in the community. When Michael goes to Italy, its amazing how even thousands of miles away, he just can't stop trying to be his dad.
The second film is about Michael dealing with getting his great wish fulfilled, and when that happens it just can't ever be all that you hope it will be. There's always a price.