I love Apple's leather case for the 6. Feels wonderful, doesn't catch on the inner lining of your pocket when you go to take the phone out, and still has plenty of grip. Plus, it's very minimal in size.
If I had to choose between the two, I would pick the leather.
Silicone can attract grit and debris and if it works it's way between the case and the phone, it will actually mar the finish. The silicone does offer more protection so if you are a phone dropper, it might be your best bet. Just remove it and clean it often.
I prefer the Sedio Surface which adds almost no bulk but lots of grip.