First, if you position your subject so they’re facing the sun (i.e., the sun is behind the back of the photographer), then your subject will be looking right at the sun! That’s a recipe for unappealing squinting, guaranteed. For this reason alone, it’s almost always better to have the subject facing away from the sun.
Next, positioning the sun at the subject’s back will provide a nice hairlight, which helps to separate the subject from the background. (This works particularly well if you position your setup so that the background is in shade, or generally darker than the subject.) That edge light is the stuff of beautifully crafted portraits; don’t underestimate the importance of increasing the appearance of depth in any picture.
Another big benefit of putting the sun at your portrait subject’s back is that the light illuminating them will no longer be specular and direct; now it will be reflected and diffused. That means it’s going to be softer, more even and generally more flattering than harsh sunlight. Not only will your subject not be squinting, they will be set off with a hairlight and illuminated with beautifully diffuse portrait light. What more could you ask for?
If you’re pointing your camera toward the sun, you’ll have some challenges to consider. First, you’re more likely to get flare. Lens flare will rob your picture of sharpness, contrast and color, so try to use a lens hood and a flag—your hand, a friend’s hand or even something like shade from a tree or other structure—to keep the direct sunlight from falling on the lens.
Next is exposure. How do you expose correctly when your subject’s face is effectively in shadow? Well, from a typical daylight exposure (i.e., if the sun was shining directly on their face), you’d want to open up a full stop or even as much as two stops, depending on how much fill light is reflected into their face. If it’s a deep dark shadow, you may have to open up two stops, whereas a strong reflected light could make the correct exposure only a half-stop up from the normal direct sunlight exposure.
Another option is to consider a fill light by way of a flash or reflector in order to illuminate the shaded face and maintain the basic daylight exposure. One benefit of this approach is that you can exercise a bit more creative control over your lighting. Add more flash, and you can stop down to darken the ambient exposure. Or, with less flash—or less reflected ambience—you can open up the background a bit. Even just a simple white reflector can make a world of difference in this type of lighting, as evidenced by the examples here. No matter whether you use a flash or a reflector, it’s this sort of lighting control that separates the “take what you can get” crowd from photographers who “take control.” It all starts with how you position yourself and your subject when faced with bright sun.