It depends upon the intensity .. but if you compare apples to apples, Mental might be worst, cause if you have mental disorder, you are free from the obligation of Namaz and Ibadat ..
under physical illness, you have to do it, even through eye movement if body movement is not possible.
So even Allah acknowledges the seriousness of Mental disorder ..
but this is just my thought, not a fact for that matter.
excellent point..i never thought of it this way..!