I think in general, if overwhelming constant never-ending mind-bending pain is the primary symptom, then it's probably fibromyalgia. If fatigue, exhaustion, weakness, worsening after exercise, or viral feelings (swollen glands, feeling sick, etc.) are the primary concern, that's more of an ME thing. Although of course, a person can have both. And a person with fibro will often still have fatigue, etc. the same way a person with ME will still often have pain issues.