I think at the end of the day, they both were equally as ruthless, but the film obviously is about Michael's life. So naturally, since there's more screen time of him throughout the entire trilogy, you get a sense that "we've been with him the whole time...his entire life."

So while we've seen dashes of Don Vito's ruthlessness, I dont take that to mean "he wasn't as" ruthless as Michael. Plus, remember-we're talking about BOSSES here, not capos, not soldiers, who(in my estimation or ignorance here wouldn't have as much to worry about like a boss would.