I too was in this position and chose Finton House, we could not be happier. We also have a summer born child. They cater to all ages throughout the year and it has not been a problem.
It has a wonderful ethos and feel and our decision was based on this and the larger year groups over Wandsworth prep. We felt by the time our children got to the upper years they would find Wandsworth prep very small and a larger year group would prepare them better for senior school. There is also a larger range of activities, after school clubs etc.