The amount of protection seems about equal. I've owned both.
I bought the leather case when I got the phone, and used it for a few days. However, it started to discolor rather badly within a week, so I returned it and got the black silicone case.
The clerk said she has seen a lot of returns of the leather case due to discoloration. The fit and finish of the silicone case also seems superior so far.
This was over a month ago, and my silicone case still looks flawless.
I'm probably going to buy a Lifeproof case when they become available, for when I go to the beach or to do strenuous outdoor activity, but at work, around the house and on most nights out, the Apple silicone case wins for me.