Tsitsipas has been on poor form of late, but he is a clay court specialist with history of doing well at Monaco. Sinner should have beaten him if his body held up, but takes nothing away from Tsitsipas.
Ruud's game similarly is set up well for clay. He was a RG finalist in 2022 and 2023. Never won a masters tournament, though, and is often mocked for being something of a 250 king. He played well against Djokovic. The only thing that really went wrong there was when the crowed annoyed Novak. He loves that, and went on breadstick Ruud in the second set. Djokovic hasn't reached the SF in Monaco since something like 2015 though.
If Ruud can use his high looping topspin forhand to attack Tsitsipas' backhand, then I think he'll win. His problem in finals before is that he's been too passive.
EDIT: Well Tsitsipas took it comfortably. Ruud was never in the match. He was rushed, and couldn't find the range with his his forehand. Nice to see Tsitsipas back to wining ways.