Mainstream or Specialist school

Hi there, my husband and I are parents to our 6 year old daughter who has been diagnosed with autism and has development delay.

She currently attends a mainstream primary school and has an EHCP (since the last year of nursery).

We are at a stage where we are considering what is best for her future education and wellbeing, she does attend the mainstream school well and has been settled in her class with the other children, teachers and her TA.  The issue is whether she is meeting targets set in her EHCP, she seems to be making some progress but not enough to meet the initial targets set.   These had been changed by the schools SENCO.  

I feel that we are a point where we have to make a decision on what is best for our child going forward, as much as she is happy going to school and the staff and kids are really good with her I am not sure she is progressing with basic skills and looking to the future, is she going to get left behind?  She also does not have any friends that she can talk too, she is verbal but struggles to respond to others and cannot have a conversation.  

Has any one else had this same problem deciding on mainstream or a special school?

I would appreciate any advice or tips on this.

Thanks x