Rafa over Ruud: It all boils down to fitness. If Rafa is fit, or close to it, he wins in straight sets. Nadal's struggles in the second set against Zverev leave that question well open to doubt. If Nadal is much less than, say, 90%, he will face a challenge in Ruud who has won seven clay court tourments, though none of them above the 500 level. Rafa's w/ue ratio is +38 so far this year at Roland Garros, very good but not great for him. Ruud's ratio is a massive +121, best in the tournament by far, indicating a player who hits a lot of winners but makes few mistakes (he averages well under one unforced error per game (.60), which is superb). Surprisingly, Ruud even has a slightly higher ELO rating than Rafa on clay.
Also weather could play a factor if the roof is closed and humidity slows conditions to a crawl as it did against Zverev. More grinding is not what Rafa needs. The longer the match goes, the better the odds on the challenger.
But can Ruud beat his childhood idol and current mentor (for the last four years Caspar has trained at Rafa's tennis academy in Mallorca) on Court Philippe-Chatrier in his very first GS final? Seems like a prohibitively daunting task psychologically.
Previously, Ruud has gone this deep only once in a 1000 level event (Miami).
30% chance Rafa in straight sets; 50% chance Rafa in four sets; 20% chance Ruud pulls the upset.