Big BIG difference in what these two terms connotate. I wish saying that someone "has Autism" would disappear off the English Language, and any other language in existance today, and here is why:
Making a statement that "I am Autistic", or So and So "is Autistic" is a declaration of being. An identification with certain inate traits and characteristics. An explanation of how a person feels, thinks, relates to the world.
Saying that "Someone has Autism" implies that this person is not well. That they have an illness that needs to be cured/fixed. It implies a judgement by the NT world about what being normal is, and how someonw that has Autism falls short of that norm, because of the Autism.
Talking about Autism in general, as in the title of this Forum (General Autism Discussion), is fine, in my opinion, but not when talking about individuals.
I feel this is a very strong distinction between the two terms, and language shapes our thinking and implied beliefs. I know not everyone feels strongly about this (and that's fine - there is stuff I don't care about one way or another that is very important to others, and I respect that) - but I just wanted to make an assertion about how I feel.