Thiem.
Both his RG runs were played against tougher comp IMO, and peak Thiem had much more power than Tsitsi.
If they played each other, What is Tsitsipas going to do when Thiem attacks his backhand?
Serve: slight edge to Tsitsi
Volleys: Slight edge to Tsitsi
Forehand: Thiem
Backhand: Thiem
Movement: equal
Mental: ...
Thiem's inside-out forehand and down-the-line backhand are much better than Tsitsi, who relies more on height and spin cross court. I don't think Thiem gets too bothered by that, he can back further on clay and unload with a longer swing.
10 match series I'm thinking 7/3 8/2 to Thiem, partly a match up issue, partly Thiem being slightly better