I’ve been toying with this dilemma as well recently – I have a 100mm stem on :?my 29er and my LBS very helpfully had a rummage in their spare parts box and found an 80mm for me to have a go with. Might be worth asking your to do similar. On the understanding that you end up buying your new stem through them though, I guess.
Haven’t yet had time to try them both out in a control experiement to see whether there’s any discernible difference.
It’s a bit of a brainache as there are so many variables, stem length, angle, bar width, bar rise, bar sweep. 😕 Impossible to try them all out.