Do I replace my slave clinder or my master cylinder?

It has a leak in the system.You need to find where the leak is and replace that part.If you slide under the car you should be able to see if it's the slave cylinder.Same with looking under the hood at the master cylinder.It could also be the line between the two.It feels and act strange because the leak has let air into the system so your not getting a complete clutch disengagement when you step on the pedal.That's also why it grabs so soon after the pedal comes off the floor.