What's the physical/mental difference between someone with autism and someone with down syndrome?

With Autism thats not always true. It depends on the type of autism they have and how server. For example: I went to school with a kid with autism. There is no way he could ever function in the world. He could barely speak. On the other hand my friends younger brother was diagnosed with autism - but is still able to function. After working counselors and learning specialist, he is more or less normal - and is now almost out of HS, and gets along just fine. He just has a few more quirks than most people.

Ok so taken from wikipidia: Autism is “ characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior” http://funadvice.com/r/3js7leip48

Down Syndrome: “Individuals with Down syndrome may have some or all of the following physical characteristics: microgenia (abnormally small chin),[4] oblique eye fissures with epicanthic skin folds on the inner corner of the eyes (formerly known as a mongoloid fold),[5] muscle hypotonia (poor muscle tone), a flat nasal bridge, a single palmar fold, a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils) or macroglossia,[5] a short neck, white spots on the iris known as Brushfield spots,[7] excessive joint laxity including atlanto-axial instability, excessive space between large toe and second toe, a single flexion furrow of the fifth finger, and a higher number of ulnar loop dermatoglyphs. Most individuals with Down syndrome have mental retardation in the mild (IQ 50–70) to moderate (IQ 35–50) range,[8] with individuals having Mosaic Down syndrome typically 10–30 points higher.[9] They also may have a broad head and a very round face.” http://funadvice.com/r/be8nlvptvit

Hope that helps.