The true answer is neither and both.
A camera alters a 3D image into a 2D image. Think about it...if you take an orange, which is round, and look at it, it will look round, it will look like a sphere. Take the same orange, take a photo, and suddenly lighting, dimensions, and other things our eyes don't pick up are forced from looking spherical to looking like a circle...essentially, it flats the object out...or "adds 10 pounds". There is a reason why that phrase exists. Cameras alter the lighting around us and the lighting around us can make buldges appear, can hide imperfections, can cause flaws to look magnified, the angle you stand can create more or less shadows, the list goes on. Flat lighting (from directly-on) is one of the most unflattering lighting schemes. However, this is EXACTLY how on-camera flash applies the light.
Don't believe me? Go to the local drama center in your town and talk to the people there. They'll tell you how lighting is both a friend and a fiend. A camera just magnifies that spectrum to capture the image in still frame. Additionally, cameras are hardly EVER taken in the height that does not alter your body shape in any way. It would have to be exact, and that just doesn't happen unless you are doing complex math calculations.
A mirror is not perfect either. It depends on your distance from the mirror, curvator of the mirror, imperfection of the reflective surface, LIGHTING AGAIN, angle, and so forth. In addition, our faces are not perfectly symmetrical. Our eyes are normally drawn to either the right or left side of a face, however, when you look in the mirror, you are seeing the side of a face you rarely ever see, so it does not give you comprehensive view of your actual looks. Because the sides of your face aren't symmetrical, this creates a familiarity with a specific arrangement and proportion of features. This is not however, what other people see when they look at you. Very few people have the symmetry necessary to look the same in the mirror as they do in a photo
Want my opinion? The perfect stranger is the best way to get an accurate portrayal of yourself. A loved one will tell you either what they think you want to hear or will have an altered view of you. A perfect stranger has no reason to lie to you and has no altered perception.
Of course, many of us are too afraid to hear what a stranger has to say, so in that case, I'll stick with the mirror. At least it doesn't flatten my features, add shadows I can't undo, and prohibit me from finding the perfect "mirror face".