In the music theory class I'm taking, I've found that even though I was originally trained on piano and have never had any chorale training, I have a better job hearing intervals if I use a sampled choir instead of some of my favourite sampled piano sounds. Not sure why this is? Or how common such perception is. Would be fascinating to find out if we've evolved auditorially to better hear distinctions in sound if the source is the human voice.
I've looked at several sampled choir VST plugins and am really lusting after the Eric Whitacre choir by Spitfire. Until I can afford that, I've been using their free choir VST plugin based upon the premium one (link; I've also really been enjoying their pipe organ samples, too).
In fact, I was having such an incredible set of personal sound revelations with the choir plugin that I tried doing a simple arragement of Chpoin's prelude in C minor. I've uploaded the result to soundcloud:
I never imagined saying this, but I think in my future studies of counterpoint and harmony, I will be spending a great deal of time with chorale music and maybe even get some formal training on the same. I suspect that learning to compose for choirs would be extremely beneficial in better writing voices for other instruments / ensembles.