I assume this is partially driven by the 29r frame geo? I ordered the 18.5" and I'm 6' 1".
This depends on your personal sizes(height, arms, torso, etc.)
If you can, draw the geometry file of the frame.
Then measure your current bike that fits you well (distance between the back of the seat and the bar, drop between the seat and the bar, ...).
And try to get the same measures on your new bike.
Using the stem length.
The stem angle.
The shape of the bar.
Some spacers to lift the stem if necessary.
A straight or offset seatpost.
And the position of the saddle on the seatpost can be tweaked too.
I think we have shorter stems now because we have longer bars