My $0.02, and it's INCREDIBLY subjective.
Ronaldo is believable, Messi isn't. You sit there in a lab & you make the perfect footballer, you'll likely end up with CR7... But if you let something just happen, it'd be Messi.
I remember an interview with SAF & John Parrot where he's talking about Messi vs Ronaldo, and on how he believes CR7 would fit into any team in any division in the world & make a massive difference, while he's not sure Messi could. I think that's pretty fair, honestly.
You put CR7 in a Villa shirt and they're 20 points clear of Wolves Not sure Messi would do that, but again, it's just theorizing.
In terms of who they are, and the clubs they play for at the moment... For me, it's not a contest. And this is probably the most controversial thing I'll say, but I truly believe the gap between Messi & Ronaldo is bigger than the gap between Ronaldo and the world class players just beneath the pair. But that's likely biased somewhat by the fact that Messi drops my jaw, while Ronaldo just keeps giving 100% to being the best player that he can possibly be. Although do note I'm not saying Ronaldo hasn't dropped my jaw, or that Messi doesn't give 100%