@Shamrock is correct about slowing thieves down, rather than stopping them. If a thief has the proper power tools, no safe - by itself - is going to prevent him from breaking into a safe. The keys are how much noise is he going to create and how long will it take?
My safe is positioned so that it cannot be pried open without first destroying a portion of an interior wall. It is also bolted to the floor. I have a home security system, adding to the multiple layers of security.
Further, the number of people who know I have a safe (and where it is) I can count on the fingers of one hand.