The boy chorister treble (soprano) is a 'head' voice which is supported by pushing lots of breath through short vocal chords. It is (for church music) very pure and free from vibrato. (this also leads to conflicts with singing teachers who tend to want vibrato for exam purposes) The girls soprano is a 'chest' voice which gives it a different timbre (different part of body vibrating or similar). If the two voices sing in unison then you can get a phasing sound as they meet. There are some pieces (mainly modern) where two choirs are used, one boys, one girls, but in most cathedrals it will be one or the other supplemented by men OR the boys will take one side and the girls the other and sing different lines.

There is also some debate as to whether girls voices should be trained so young before their bodies have developed to support the voice. In the case of boys the problems seem to arise if they keep forcing their voices when they start to break.

Choral evensong is one of the glories of music, available for free, performed at most of our Cathedrals most days. Well worth attending, even for non-christians. The standard is almost always high and the music varied.

All the best

Steve