What makes you comfortable with the mm wave system? My understanding is that more research is needed on how millimeter waves affect cells. http://thz.caltech.edu/
While more study is needed with any system it is my understanding that millimeter wave radio waves will not penetrate very far beyond the surface cells while x-rays will pass through the entire body mass. Just on the raw mass affected by the device the millimeter wave system seems less risky.
The x-rays don't pass through the entire body. They go only into the skin. That's actually what's most concerning about them. Because they don't pass through the body, they are concentrated entirely in one layer which receives a much higher dose.
