Grippers have a softer urethane, so they grip more, but a little slower. Sliders have a harder urethane, so they slide more in park, but are faster.
Grippers are better for park, sliders are better for street.
Gripper in the back and slider in the front is a good all around setup.
These are all assumptions but I'm pretty sure I'm right.
Edit: also, this is the wrong section for this. I know your new so I'm not gonna get pissy but next time, post a thread like this in the Wheels/Bearings section here:
http://www.scooterresource.com/forum/forumdisplay.php?f=90
Also, there is a search button, as I'm almost positive theres been a thousand threads with this exact question. For future reference, you should search every time before you make a thread, to see if there has already been a thread that answered your question.