Cost and aesthetics are primary concerns when choosing the best over-mantel mirror, but you will also want to consider the frame materials, weight, and size of the mirror as well. Measure your mantel carefully as well as the space above the mantle where the mirror will be mounted. This will give you a better idea as to how much space you have available for the mirror, and remember that most mirrors need to be mounted to the wall above the mantel. Be sure to find out what the wall is made of and how much weight it will be able to support.
The aesthetics of your over-mantel mirror should fit well with the rest of the decor in the room. This is not to say you cannot buy a more decorative unit, but you will want to consider how that mirror will look with the overall decorative scheme of the room. The mirror should be a focal point and a point of conversation, but you want those conversations to be pleasant ones. If the room is decorated simply and sparingly, choose an over-mantel mirror that mimics this style or strays from it only slightly. A more decorative room may benefit from a highly decorative mirror.
The frame material of the over-mantel mirror will have a significant impact on the appearance of the unit. Wood and metal are most often used for the frames of the mirror, and different types will lend a different appearance. The finish of different woods, for example, can change the color and tone of the frame, and painting it can change the appearance entirely. Steel will have a different aesthetic than wrought iron, and each type of metal may fit equally well with different room aesthetics. Remember that some metal frames can be quite heavy, so take this into consideration when purchasing.
Consider how the over-mantel mirror will be secured. Some mirrors are light enough to simply rest on the mantel itself and lean against the wall, while others will be heavy enough to necessitate an anchor to the wall. Regardless of weight, it is always a good idea to somehow anchor the mirror to prevent the unit from falling and breaking, and potentially causing injuries to bystanders. You will need to consider the overall strength of the mantel as well as the materials used to build the wall above the mantel. Securing a unit into brick is a more involved process than securing a unit to wooden studs.