Glad you made a decision in regards to the horn of choice for Christmas Service. At my church for Christmas, and Easter for that matter, any and all of those horns would be welcomed. I play tuba every Christmas and Easter, along with at least two electric guitars, a bass guitar, a dulcimer, a harp, various recorders, flutes, clarinets, piano, bongo drums, electronic drum set, and a synthesizer hooked into the electronic organ. We really only use the organ for its MIDI controller abilities for bigger services. When I first started playing, we had a few more people, including two trumpets and an organist.
At my church the music director gives all the musicians copies of choral scores with chords written in. We are all free to play what we feel like. I will routinely play chord roots while the bass guitar winds up picking the bottom note of an inverted chord... adding random dissonances. I will, at times, play whatever the men in the choir are singing, or the written hymn melody or harmony. The closing hymn for Christmas is almost always Joy to the World. Our version is in C, with an SAB vocal scoring. it lies nicely on a C tuba when I play the baritone voice part down an octave, and at fff. No one seems offended. Most of the congregation seems appreciative of a loud tuba. Now to just get some more brass to fill it out!