DESCENDING FRom F370 to F350 due turbulence if I am not in an airspace committed separation by MACH I would revert to SPD personally primarily because I would at constant pitch and Spd mode descent my F/F will reduce in that 2000' ft band might as well take advantage of that. Secondly spd has more specific selection of spd down to the single knot. & thirdly MMO excedence probability would be lesser compared to Mach mode descent if a high v/s is chosen to descend. After level off WIL be back to MACH. However on a given day, the scenario will dictate the best selection
