"I wanted him to learn to socialise with groups of lots of different children without me being there, rather than the same one or two every day"
This is a completely fair point and a really popular reason for choosing a nursery BUT a good childminder will make sure your DC has lots of opportunities to meet other children.
Fir example, this morning, we went to the playgroup across the road so lots of children there.
Tomorrow we will go to another childminder's house to play, so that we be another 3 children.
Last week we went to soft play...and so on.
There are LOADS of opportunities to mix with other children and it's a little unfair to assume that childminders don't bother with that.
I also feel that the close friendships with the children and the childminder, as well as a bigger age range and meeting other people in the community offer more social interaction than being in one room with the same people all day in a nursery.
I've worked in nurseries and the children tend to bounce of each other. There are so many that to each other, they sort of become "part of the wallpaper", IYKWIM.
Whereas a (good) childminder would offer a more varied level of social interaction. There are times when we are out with loads of the kids. There are time when it's just me and one child at home.
There are a lot of little social interactions that happen everyday that don't seem important but all add up. For example, getting on the bus and asking for ticket, or the bloke who runs the fruit and veg shop who always smiles and says hello. Children in nurseries miss out on a lot of that.