the large stroke is going to hit harder -better for 23+ for large fields of grey tones for say a large back leg pieces, for solid block colors,whip shading & for 8-18 liners
the 3.5 is good for every thing else 1-7 liners,shading,blending, layers etc
they both can run can run slow to fast unlike alot of other liners out there that lock up are tend to vibrate to much when volts are cranked