I was planning on using a Power Support Crystal on the front and back of my 4S (I have one on the front of my 3GS and love it), but I'm afraid it will be too slippery. So I started looking at skins like the SGP Skin Guard (Deep Black Leather) or the Phantom Skinz matte. Do either of these provide some "grip" or are there any other skins I should be looking at? Thanks!