It is very much a personal preference thing, watch the racers and you will see that some of them choose to use a leather slider, Mick Doohan for one springs to mind, and others use a plastic slider.
One other frivolous but important point to mention about leather sliders and that is that they look as cool as heck when they have been worn through a bit and they are all scorched and frazzled and the layers are exposed. As you can probably tell, leather sliders would be my favourite :-)
GP leather sliders are handed left and right, for a really nice fit.
If you require your sliders to have the Velcro reversed, IE: with the loop backing rather than the hook, please remember to let us know. there is no extra charge for this service.