of which specific impulse is most important 2)payload=/=take-off weight (do u hav hi school physics?)

A simple example: A single stage to orbit rocket has take-off weight=170 ton of propellent+30 ton of frame + 2 to 3 ton of LEO payload. Even a 5% improvement in specific impulse could mean 8.5 ton increase in payload.

Another simple example(compare 2 configs of 1 rocket)

config 1: take-off WT=100 ton, 1st stage engine thrust = 133 ton =>net upward net thrust to WT  ratio= 1:3

config 2: take-off WT= 102 ton, 1st stage engine thrust = 200 ton =>net upward thrust to WT=0.96:1

Config 2 has ~ triple the takeoff acceleration but the burn time cut by a 1/3, still preferrable for the first few 10s of seconds but

a)after that the rocket weight will be balanced by centrifugal curving of the trajectory, very hi thrust not needed

b)More engine thrust more expensive though added-on solid fuel side booster(cheaper) might help