I think this is going to have a lot to do with where you are. In my area, preschool is offered as an intervention for special needs kids, but ONLY for special needs kids. There is no "mainstream with an aid" available, because there is no publicly funded mainstream preschool - we all pay that from our own budget. If your child needs services, you go to the special county preschool; if your child does not, you choose - and pay for - your preschool. Or work with your child at home.
Look around and see what the options are in your area. If what you want for your child is not available, then you have decide how important you feel it is for your child and what you are willing to pay to get it. Once your child is in mandated elementary school, there should be more choices. But most places don't require preschool and, thus, the options are more limited.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).