Is one type of mutation worse than another in terms of disease severity? 

As we know, not all patients with Warburg Micro Syndrome (WARBM) or Martsolf Syndrome have the same symptoms, some children are more severely affected than others in certain aspects of their disease. We understand from research such as that published by Dr. Mark Handley and colleagues in 2013 (summarised elsewhere on this website) that severity of disease correlates with the extent to which the function of any one of the proteins that directly work in the RAB18 pathway is affected. Complete loss of function of one of these proteins is associated with severe symptomatology and WARBM. Disrupted, but not complete loss of, function is associated with milder disease symptomatology and Martsolf Syndrome. Therefore, it is not a case of one type of mutation (nonsense vs missense) being worse than the other, but rather how the mutation affects the function of the protein produced.