I don't know enough about the specific tests to suggest one over the other. But I would think that the child would need the right examiner not just the test. The tests are only as good as the examiner is at both administrating and interpreting it. And testing a child on the spectrum, no matter what the diagnosis, takes a skilled examiner who is very familiar with both the tests and how to give and interpret them.
I would suggest finding an agency or psychologist who states they specialize in testing kids with ASD. Then ask that person about their experience with gifted kids with ASD. You should know by their responses whether they are likely to even have a clue about the whole thing! I've run into psychologists who think that the relative strengths are meaningless simply because the child DOES have autism. Better to find someone who understands the relative nature of the numbers from a test and how to use the child's strengths to support the weaknesses.
The school may be right, that any test given wouldn't really be meaningful due to his inability to follow directions. But perhaps you won't know until you try. I would suggest private testing thought, lest the school test, get bad scores and use that against the child.
