Weirdly, the overs market is one of my favourite to utilise for the latter stages of Grand Slams on the mens side particularly (both semi finals went overs). The key behind overs & unders isn't so much how good the players serve is for clay, more so "are the players evenly matched & will they fight from behind?". For this one I think both questions are answered with both players often raising their game from behind. In both their last H2H matches they've gone the distance, with Tsitsipas fighting back from 2 sets down before going down in 5 at the French Open last year.
Tsitsipas is exhibiting such a high peak level when he's on his game currently and with the afternoon conditions of the final compared to the evening for Djokovic vs Nadal yesterday, he'll be able to benefit from the extra heat which will give his forehand a bit more pop through the court. The worry for unders would be Djokovic defending to well and wearing Tsitsipas down but I think it's less likely because of that factor.
Overall I expect it to be a high quality affair and I think over time Djokovic metronome type style of play of consistency will probably mean he'll be stronger in the end. But I really wouldn't be surprised to see a 5th set here and I hope it's a classic!