Ultimately, for rehearsal purposes, I think either instrument can be helpful. The sound of the organ with its winded and sustained tone is arguably more vocal and certainly less intrusive on choral timbres. On the other hand, the percussive nature of the piano can be beneficial in communicating complex rhythms and merely 'keeping time'. In the final analysis, though, there can be no question that the best teaching aid for singing is the human voice. Whatever instrument one uses as a rehearsal aid, one should be able to demonstrate vocally every musical line, nuance and diction... most particularly with chant (never ever try to teach chant but by vocal example because the soul of chant cannot be illustrated nor apprehended but by the human voice). Every choirmaster, of course, hasn't the 'voice' to do this, but the extent to which he or she can will make all the difference in the results achieved: don't just describe what you want or plunk out pitches on an instrument.. demonstrate with you own voice the sound and musicality you want.

I would like to amplify what I said above: teach by singing and wean your choirs from instrument dependency. Teach them to sing with their ears and to hear within themselves the pitches, rhythms and nuance that make music music. A piano may ultimately be self defeating because it introduces an essentially non-vocal element, timbre, into the psyche which will never be used in liturgical performance.

I'm with Earl--I don't think that I know exactly what you are asking. Could you clarify?

Well, people here are not using their brains (which are gifts from God)....or maybe I wasn't clear enough.

I was asking about the organ and vocal exercises like singing scales. People say that the organ is more similar to a human voice. Does this mean that we can use the organ when doing these exercises?

You can use a ukulele, as long as it's in tune.

Piano/Organ doesn't make all that much difference for personal practice. I would be more inclined to recommend piano since you can hit particular notes harder for emphasis when checking tuning of certain phrases, which you cannot do on the organ.

For choir rehearsals, I would tend to recommend the organ, simply because many older organs are not tuned to A=440. The organ in my church is tuned to about A446/A447 (varies depending on the weather). We got around tuning issues by using an electric piano in the rehearsal room with the pitch adjusted to match the organ.

You can use whatever works best! If you have both a piano and an organ, I would expect the piano to have some advantages in exercises (rhythmic clarity, pitch stability over time).