FYI, those dimensions listed are for the stem extension (horizontal part), not the stem length (vertical part).

This is still a tough question, since you're changing the bar shape completely. Usually, when switching to upright bars, you want lesser reach. So I'd say to start with the 90mm. The trekking (butterfly) bars have hand positions both further away and closer than traditional MTB flat bars, so you have a lot of flexibility here.