What difference is there between the downward velocities of the skydiver who jumped out of the

helicopter (with no forward velocity) and the one who jumped out of a plane

There's a lot of info skipped in this question, so I will make assumptions to answer it. I am assuming the two skydivers are considered projectiles; the helicopter and the plane fly at the same altitude.

Answer:

There will be no difference in the downward velocities. Both skydivers are subject to gravitational acceleration, which is the only force acting on them (i. e., is the net force). Gravitational acceleration is the same for all objects moving in a projectile motion. Their velocities as function of time will be described by

v (t) = (gravitational acceleration) * (time)