I bought a silicone case today, and I have a leather one on its way.
Personally, I was surprised at how soft and slippery the silicone case is; I thought it would be grippier and have a more rubbery texture. It almost feels powdered. I really wanted the case to be as grippy as possible because I feel like with the Plus I'm going to be juggling the phone a lot to reach different areas of the screen when doing things one-handed.
I like the look of the leather case a lot better, but I can't comment on the grippiness of it yet. I'm hoping it's even just a little better than the silicone case. One nice feature of the leather case that I believe is going to really make it the better of the two is the fact that it will age and wear over time, giving it a unique character. I love leather; so it's the case I really want.