Hi OP
I too had this dilemma. Well I didn't even know special needs preschool exist!
DS is 5 and will hopefully be going to a specialist school this September. He has ASD and severe oral motor and verbal dyspraxia. As you can imagine he has very limited language.
Shazzarooney DS is very sociable, has many friends and despite his ASD, can cope with the sensory demands in a mainstream school. But as 2boys.. mentioned, I find out that DS was just "babysat", don't get me wrong, his teachers and TA's are lovely, the kids are lovely to him. But the school hasn't got the facilities to accommodate a child with speech, language and social communication difficulties.
OP, you need to visit as many schools as you can. I looked at 14 special schools and 7 mainstream schools. In honesty, I wasn't impressed with the majority of special schools that I saw. However in the end, after visiting the number of schools. I picked a very specialised independent special school. I had that gut feeling that this school was the one, whereas in my mind, ideally, I would of liked a mainstream school.
I am a bit worried about his social needs, but I plan to get DS involved in many community actives, such as; football, gymnastics, brownies etc.
OP, visit as many schools as you can, look on reviews about the schools, ask parents, ask questions and trust me you will have that gut feeling that your making the right choice, whatever type of school you feel could meet your DS needs.