Try to do some test rides on similar bikes..e.g. Specialized Epic Is similar. M in Speci corresponding roughly to 909 S size.
When you know what feels okay, it's better to choose.
60mm doesn't feel sluggish to me... but that is personal choice. I think 40/50mm is not stable enough for me..
This can be a difficult choice even with a test similar bike. If you like the fit, then you need to build the same, taking into consideration the difference in frame geometry.
My test bike was a Carbonda 936 L-size (same geometry) with 100 mm fork, 51 (i think) mm offset, 80-90 mm stem. The fit was very low and stretched, the steering was very sharp, the bike itself was kind of sluggish.
My friend decided to buy M-size, I bought a size L anyway. We are both 180 cm tall.
To avoid negative moments, I installed a 120 mm fork with offset 44, stem 35 mm (also i bought 45 mm), riser handlebar 760 mm, spacer ring under the stem. In my opinion, it turned out perfectly for my riding style.