What is the difference between mildautism and severe Asperger syndrome?

No good measures. We do not have good measures of severity for autism or asperger's disorder. "mild" austism usually refers to cognitiviely normal individuals with social problems some oddities in speech, and unusual or odd behaviors. Asperger's disorder has minor to no speech problems but impaired social social skills and odd behaviors. The two could look quite similar, and both can cause severe impairment.