oh boy...a slow weapon -> higher chance to trigger a proc with every hit. as sinister strikes are additional hits, that are limitated by energy you will generate more procs with a slow weapon.
deeper:
a slow weapon does less hits per minute, so the % chance to generate a proc must be higher to maintain the PPM rate of the specific enchant.
as you will have the same amount of sinister strikes with both weapons, the slower one will generate more procs in the same amount of time.
basic PPM may be 1, a slow weapon will generate 1,3PPM while a fast one only generates 1,1PPM...(no real values)