We've just had this very same decision to make.
Pre school or school nursery.
He loves his pre school, he is already in a year above than what he should be, so at 3 is playing with children age 5, so he has come on enormous amounts. My worry IS he is going to be back in with children the same age as him.
But..
We've chosen nursery school.
Although he was settled and loved the pre school. The nursery school seemed like the best decision for us.
Simply because, they have a lot more carpet time, reading time, learning time.
They also use the traffic lights system for behaviour. Which I think is fab. They also have to wear a school uniform, and to me I thought it would help him get into the routine and use to the school setting for when he starts full time.
A pre school nursery focuses more on play. The managers admitted themselves at pre school, it is all about play!! They don't do the same sort of curriculum that school nursery have to do.
Hope you make the right choice op.