There a quite a few things to take into consideration when looking for a gun safe. The biggest are what are you using it for and how much security is needed. Here are a couple things to keep in mind:
You want a safe that not only fits all of the guns/rifles that you current own but also ones you may potential purchase. A safe is a big investment and you do not want to run out of space. Better to spend an extra $100 in the beginning than to buy another safe.
Do you need it to be fire resistant? Most reputable safe companies make their gun safes fire resistant, but there are gun safes out there that are little more than gun boxes with very little safety to them.
Can the safe be bolted to the floor? You want a gun safe that will be bolted down for a couple of reasons. One the safe cannot be put on a dolly and stolen. Two, it won't tip over either hurting someone or giving a thief leverage when trying to pry the safe open.
Bolts on the top and bottom of the safe door. You want a safe that has bolts on all sides of the door. This helps prevent a safe door from being pried open.