The leather case. It looks classy and has a more price-appropriate feel. Silicon cases should never be as expensive as Apple charges for them, and they make the phones look more cartoonish ala the iPhone 5c.
I got the Black Silicone and Black Leather. Also have AppleCare+ as well since I intend to keep this phone 2 years.
I'm currently rocking the silicone for a few reasons:
1. It's more durable and better at protecting your phone.
2. Thermal properties are better when it comes to dealing with heat.
3. It's actually more dark grey than Black so it compliments the phone well.
My black leather case is still brand new in box and I'll probably start using it in a few weeks or so.