look at the logistics will it be easier for you to take both boys to the same place? if you have to take DS1 to school anyway, then you might as well take DS2 to the nursery there. That's how I would look at it, anything for an easy life!
I have a similar position with my DD. DS is going into year 1 next year and I am moving her out of her current day nursery to go to the school nursery.
For me the practical benefits are huge, plus I think it will really benefit in terms of getting used to the school and making friends before she starts school. DS started into reception WITHOUT going to the school nursery and although he has settled really well, I do think that he would have settled better and more quickly if he'd been to the school nursery first.
I agree with a pp that 3 days is really not long enough to settle in - the staff at the school nursery are suggesting that the whold period up to October half term is considered to be "settling in" time.