What also separates the two for me is their role within a team. Stupid analogy ahead.
Ronaldo is an incredibly deadly weapon, but he's little more than that. Real have won finals where Ronaldo has been quite anonymous, just because the fear of giving him space to swing his sword. We didn't even seem to have to do much to stop him last year, and he was a passenger in the final versus Atletico.
Messi, on the other hand... He's not just the deadliest weapon of Barcelona, but he's also the heart & the brain of the side, especially since Xavi left. Everything goes through him. Messi can't afford to have bad/anonymous games like Ronaldo can, because he is at the very core of the team. Even if he doesn't score or assist for whatever reason, the ball still has to go through him, & his range of passing, vision, and consistency for this is on another planet altogether. You can say it's all down to having great players around him, or that being at Barcelona allows him to specialize himself so well - There might be some truth to that, but at the same time, the team he's playing with now is hardly incredible, and he's performed at a top-top level under several different managers.
That's not to say there haven't been times where Ronaldo hasn't done freakishly well while Messi was perhaps falling below is incredibly high standards somewhat, but overall? I still maintain that the difference between Messi & Ronaldo is bigger than the difference between Ronaldo & whoever you consider third on the list.