Hale prep is popular because it is co-ed, as mentioned above, it is often easiest to get a place if you register at birth.
Forest is also co-ed and I think also harder to get into these days. I know of several children who have move there from other local preps and considered it myself. It seemed lovely, but I didn’t want to uproot my eldest at the time.
St Ambrose has also recently gone co-Ed. They have few girls at the moment, but I know of several starting.
Bowdon is regarded as a bit of a hot house. I’ve heard that several kids leave in juniors to go to withington, so it can be easier to get places at that stage.
Loreto has been nurturing in the past, but isn’t as great at the moment. The competition with St Ambrose may encourage them to sort things out. Their facilities aren’t great either. Also, the girls have to score higher than state school girls in Trafford to get a place at the grammar.
The prep schools in Alty are not on a par with the Manchester schools, they have fewer facilities and extra curricular.
If your going for AGGS, then Bowdon is your best option, but I would also think of a back up plan. With the exception of Wellington (which has a very small catchment), the other Alty options aren’t great.