Flash or Exposure compensation? What's best?

When using my DS2 and a diffuser, the image is often a little dark when the subject and/or background is a light colour. Obviously the camera is trying to make it a mid-tone.

What is the correct way to correct this?

Is it better to alter the flash compensation (I have a Sigma 500 DG Super) by, say, 1 stop and/or the camera's exposure compensation by a similar amount?

Any guidance would be much appreciated.

John

I would bracket exposures. I think generally with a diffuser you might want a higher key image so overexposing by a stop might get you into the right area but a lot will depend on the subject matter.