We took DD (now Y5) out of a mixed class primary last July, (94 children on role), and she now attends a school with one class per age group. It has worked out really well for her. She is good academically, and we have always encouraged her at home, (DH is a teacher!), -and is predicted to get Level 5/6 in all the KS2 tests next year. Before she moved, we discovered there were a lot of things, (particularly maths) that had never been covered in her old school, so she had a fair bit of catching up to do over the Summer.
In the mixed class school, there is a tendency to do fairly well when the children are in the younger group, as they are working with older ones, BUT when they are the eldest, they aren't stretched. Each class also has a Teaching Assistant, who do a lot - taking over one age group for a particular lesson/project while the teacher deals with the other.
From talking to other parents, I would say that things are the most difficult in Y3/Y4, - how do you differentiate for literacy when some children are reading The Hobbit, and others are still ploughing through Biff & Chip!?? Year 5 children also tend to get put aside in favour of Y6, who are building up for their SATS.
Friendships can also be a problem, as some children pair off quite rigidly, and others can be a bit stuck. Parents saying that their children feel lonely/have no friends is a well worn topic of playground conversation!
DS1 is still at the small school, and currently waiting for a place at the larger one. He is happy enough, but rather jealous of his sister, and the better facilities, school Clubs and trips that she gets to go to!
I asked DD if she enjoyed her new school, -"It's really nice, but I do have to try now!" Sums it up, really.