Mainstream reception?

Hi, I'm fairly new to all this and I'm guessing this is a pretty common query, but I am in a total muddle about where is best for my son to go for reception. He has an EHCP plan with funding for full time one-to-one help. Our local, brilliant, welcoming mainstream infant school will take him or we've also been given the offer of a place at an ASD unit attached to a mainstream primary. I'd love him to have the chance to be in our local mainstream, which his sibling goes to, esp as they've been so welcoming, but worry that ultimately he won't be able to learn as effectively there as in the unit. Is it worth giving it a year in mainstream or will that just set him back if it's not right, and am I mad not to take a place offered in a unit as they can be so scarce? I'd welcome your experiences, even if just to add into the melting pot of things to consider. Thank you.