Using exposure compensation and adjusting ISO are two different methods for tweaking exposure. If a particular image is coming out dark, you might use exposure compensation to "open up a stop." Depending on what exposure mode you're using, this may mean using a slower shutter speed or a wider aperture.
If you adjusted the ISO one stop more "sensitive" instead, your image still would be dark. This is because you've changed the sensitivity of the camera to allow a different shutter speed or ƒ-stop that's automatically chosen by the metering system. So the camera would adjust exposure to compensate for the "faster" ISO setting.
So you set ISO to control sensitivity of your camera to different light levels. You set exposure compensation to affect the actual exposure of a scene, making it brighter or darker.
When choosing an ISO setting, consider this:
•?Set it to the least sensitive setting possible for the conditions; this minimizes noise in your image. But the noise reduction done in-camera these days is phenomenal, so don't let high numbers scare you, either.
•?Don't sacrifice image clarity to minimize noise. If you find yourself using slow shutter speeds and handholding the camera because you're using a low ISO, your images will suffer. True, they might not have noise, but they'll have blur, which will be more noticeable.
•?Set the ISO for the environment and use exposure compensation (or aperture/shutter speed in manual mode) for each image. When I walk into a room, I decide which ISO I want to use, then check the camera to see if the meter tells me I'm in the range of aperture and shutter speed I want. Once ISO is set, I leave it alone and use either exposure compensation, or if the meter is in manual mode, aperture or shutter speed to control exposure.