Ben,
Most importantly, this assessment that the soprano is more liturgical than the alto are by folks that do not play the saxophone. The simplest way it can be explained is that some consider the instruments that are a normal part of a symphony orchestra to be more liturgical (in the more orthodox denominations) than those instruments that are normally considered to be part of a “Band.” Orchestral equals more liturgical and Band equals less liturgical. When it comes to the various types of instruments, most folks make that association. For instance, at the Cathedral tympani are in and drums are out. Yet it’s all percussion. This association is made by folks that are not musicians or have a tendency to associate liturgical music with traditional music. Traditional being defined here as classical Western European. The Vatican has plenty of classical Western European.
It was somewhat surprising one time when a music director told me that. A gospel selection was planned for the following Sunday, and at rehearsal it was suggested to him that an alto saxophone would be more fitting to the piece of music. He still wanted the soprano because it was more “liturgical.” When I asked him he said that the timbre and the range of the instrument was more liturgical.
Also, to some folks it can be either a visual thing or just knowing that it’s a saxophone and the fear that one is going to rip into “Harlem Nocturne” for a communion meditation. It’s their mental association with the saxophone.
The soprano is in the same range as the flute, violin, trumpet, so there isn’t that stark contrast associated with the soprano as with the alto or tenor. Traditional Organist’s have a definite preference for the soprano, if they are willing to use the saxophone at all.
And then there are some folks that think the Organ is the only liturgical instrument. Before Vatican II, the Organist position was a full-time job. Not so much anymore, unless the parish aspiring to be a mini-Cathedral. Your question also gets into the rift between traditional and contemporary music in the Catholic Church. And that’s a Pandora’s box I’d rather not open. It’s a whole culture war. These folks think that anything besides the organ and the choir is heretical. They wished Vatican II never happened.
But then, we are still, as of this date, pioneers and guest's on someone else’s turf. It’s also varies largely from parish to parish, depending on who the music director and/or the Pastor is and what their preferences are. Saxophone players are not allowed in every parish. Campus ministries tend to be far more relaxed. So, if a parish wants you to play, it's a blessing and not the norm.
But then, none of these folks play the saxophone so they are coming from a different perspective altogether. As far as the folks on this board, whether soprano, alto, or tenor – it probably all sounds very liturgical to us.