Most of the time I will say "I am autistic", however I also often say "I have autism". I know that some people have a problem with the latter, saying that it's not something you have, but something you are- it's a part of you. But the way I see it, I can also say "I HAVE brown eyes" (I am brown-eyed), or "I HAVE a shy disposition" (I am shy), "I HAVE an interest in wildlife" (I am interested in wildlife).
I think both are neutral, I have no real preference.
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clarity of thought before rashness of action