I had to choose between the 2 for DS when he started school. He's in yr1 now.
We applied as usual to our favoured ms primary (he already had dx asd and 1:1 in nursery). We had met the head and liked the school, and told them about our son's needs etc.
Then got a phonecall from SEN team, advising to go and look at this asd unit. He was assessed by a panel for a place, and awarded one. There are only 6 places per year.
My ds is quite high functioning but his main struggles are social integration. When I looked at the other children there, he would have been at roughly the same level as one other boy, functioning at a higher level than the others.
I was quite keen, therefore, that he would have a lot of access to the mainstream classes. In my view, learning social skills would be much harder in a group of other children with social difficulties.
I got the feeling from the unit that they were quite rigid in their practice. His access to ms would not necessarily be according to need as they are not staffed 1:1, but when the dc went to ms classes they were accompanied 1:1. They were inflexible about a part time start, which I wanted.
I ended up deferring his entry to school, then requesting a split placement between the unit and the local ms, working to a transition to ms full time, then as time went on and he progressed all the time, going for a pt placement at local ms starting Easter and going ft in yr 1 in Sept. He has a 1:1 full time.
The unit was miles away and he would have to travel by taxi, with other boys from the unit. The school is 3 minutes walk away.
At the local school he has made local friends. When we go out, we see children who know and acknowledge him. At the park etc there are often kids who know him, it makes a big difference.
I could defer his entry and keep him at nursery (3 days with 1:1) because he was not 5 til the summer and I had the support of the nursery. If I had not been able to do so, he probably would have gone to the unit because he wasn't quite ready for ms then. He matured a lot in those few months.
It really depends on your child, the unit, and the other options available to you. I would say look carefully at all options available, take your time to decide, ask to go back again if you want. Listen to the professionals, but remember you know your child best, you know what priorities you have for them, and what your family circumstances are. Trust your gut. Never undervalue your own intuition.
Good luck, it took me a year to come to a conclusion and it made my head spin, but I am confident we made the right one.
Either may be 'ok', both will have challenges, but they are likely to be different challenges.