Hi BIP, I'm going to say again that I was pondering very similar questions a few months ago, not surprising as our DS are a similar age!
I decided if I was going to read up in detail on all the different approaches, I still wouldn't have made a decision by the time DS was due to start school, so we went for the approach which was shown to be the most effective according to the studies that we read, and that was ABA. Our programme has been running for 5 weeks now and we're delighted with it. DS loves his tutors, is more confident to try new things, and very keen to participate. His language is coming on in leaps and bounds.
Son Rise didn't make any sense for us as DS was already very much in our world (agree with JimJams here!); he had good play skills and a bit of language before we started. I felt he needed intensive 1:1 teaching, lots of repetition, and to be "blasted with language", as our consultant put it. My personal view has always been that DS needs to be challenged (in as kind and patient a way as possible) rather than accomodated, which I feel is what a lot of other programmes do.
We decided to turn the provision offered by the LEA, which was 7.5 hours of SN nursery per week. My main reason for taking him out of there was that I had no idea what they were working towards with him (despite repeated requests for more information), so couldn't work on the same things at home. Now, all I have to do is look into our ABA folder and I know exactly where he is at the moment. I don't think generalisation will be a problem at all.
I would also say, if your DS is happy at his nursery, and you feel he's learning, then I'd leave him there. Being able to learn from normally developing peers is a big bonus, which he probably wouldn't have in the SN unit?
(my DS was also happy at his MS nursery, but not really learning much, so we've taken him out for the time being, but will soon send him again with one of our tutors accompanying him as a shadow)