The nursery seem to be piling on so much pressure for him to start school this August - 'the nursery drafted in a headteacher who felt he was more than ready and I could be doing more harm than good in keeping him back' - that I am wondering at their motivation.
I don't know anyone who has regretted defering. My primary teacher friend defered her exceptionally large December born son and he did very well - now at university.
I didn't defer and have mixed feelings about it. It was certainly not all negative. DS did not have access to a proper nursery school and I felt he had outgrown playgroup. Also all his friends were starting school. But I can certainly relate to difficulties with sitting still, listening, the fine motor skills needed for drawing and writing, and I have to confess that his behaviour wasn't great for the first three years.
The things I would consider before making up your mind are:
Do you think he would enjoy another year at nursery, or does he seem to have outgrown it?
Does he like sitting still and doing quiet activities?
Is he good at personal care - dressing, undressing, going to the loo without having to be reminded - that sort of thing?
Are his particular friends going to school, or will they still be in nursery?
If you do make the decision to defer, I can't see much point in changing nursery, although they seem to be making you feel uncomfortable about keeping him there.