I disagree. A Part III where Michael Corleone acts like had in the first two movies wouldn't have been an end to the trilogy. It's like Darth Vader never turning good at the end of ROTJ. His character comes full circle. He learns to repent his mistakes. It makes for such a more powerful ending to a trilogy.

For example, what ending is stronger...

The ending to Scarface where Tony Montana plugs along, never changes, and eventually dies having learned nothing thus causing no emotion from the audience. A cool ending, but really, what's the point?

Or the ending to The Godfather Part III where a character who had been ruthless before, comes to terms with his life, tries to change, and ultimately fails, resulting in one of the emotional endings to a movie, period.

Which one is better and more powerful?

I don't even think we need an aswer to that question.