Stalin indeed never faced any real threat, but he also didn't rule as long as Mao nor as unilaterally. Stalin always made sure that most of his decisions were popular with a fairly large segment of the population.
Yeah, I think it turns into apples and oranges the more detail one goes into.
Mao was definitely more reckless, Stalin more paranoid, and it makes sense that Stalin's position would be more secure barring an assassination or something.
