You don't say how old your DS2 is? Is he starting reception in September?
My DS2 is 3.7 and starting reception in Sept. He goes to the pre-school that school is attached to, rather than to a nursery. He really likes it. Over the last couple of months I have seen a number of parents move their children of similar ages to the pre-school because they'd 'grown out of' their nursery, and they are all responding well to the structured environment of pre-school.
It all depends on the specific environments, and your child - but to me the benefits of the school pre-school are - in Sept he will be familiar with the school environment and will be moving up with a group of friends/familiar children - the structured activities suit him better than a free-play environment - I also have an older child at the school, so easier for me to drop-off, pick-up in one place - he is ready for a bit of learning, though they are very age-appropriate (my older DS was old for his year, and was writing his name/recognising letters etc. at this stage in pre-school, whereas my younger one is not doing this yet, but is enjoying phonics sounds etc.
The pre-school will be used to the whole spread of birthdays - every year there are some summer-borns! I worried about this aspect with DS2 too, but it seems needlessly.
You do really need to go and look at them both and chat to the staff.
Preschool with ratio of 1:6 or school nursery with 1:10 ratio for summer born boy? Anyone got any advice?
older DS goes to school that nursery is attached to, but never to a different nursery so we don't know either of these settings. There is more space at the school nursery and some of the things to play with look a bit more inventive, but there are fewer staff and they are spending some time trying to teach letters and numbers so maybe less time for nurturing and playing.
....I have to decide to day as it is the deadline for the accept/ reject offer form for primary nursery....help!