All schools are different. There are HUGE differences between one mainstream school and another, and HUGE differences between one special school and another.
My advice is to apply for a mainstream REception place now, in the usual round or applications, so you definitely have a place somewhere, come what may. IME special school places are like gold dust, so if you decide prefer one of the special school, it still doesn't mean you will get a place there. If you do choose a special school, and do get a place, then you can let the mainstream school know at that point that you will no longer need the place. If you don't have a place though, you won't get one after the places are allocated if you don't get the special school you want.
Alongside that, you need to press the EPS. If your EP assessment has been 'repeatedly' postponed, then I would make a record of that and contact the management team. If that doesn't get you anywhere then google SENDIASS + your local authority and contact them. They are there to support parents navigating through the system, or, more honestly, having trouble with the system. The whole 'applying for an EHC Plan' takes many months, and, whilst that is going on, the special school places are filling up.
Thirdly - alongside both of the above, go and visit several schools - different specialist provision, different mainstream, and maybe resource bases if you have any in your authority. Then you can express your preference based on actual knowledge of a particular school, not a generalised idea.