My favorite of the pieces to sing remains "Leonardo Dreams of His Flying Machine," because it requires turning on a dime every few pages; I also adore the way Whitacre integrated Renaissance styles in one section, then uses the choir for onomatopoeia, or whatever it's called when voices imitate sounds when they're not using actual words to do it. Hey, there're performances of it on YouTube and imeem! Some of them quite messy in places, though, and of course the sound's a bit tinny.
My other favorite is "Five Hebrew Love Songs," which is just gorgeous. And for me, "When David Heard" is the biggest technical challenge, as we have long, long phrases of repeated triplets in wide intervals at various volumes that are quite tiring, especially when it's loud, loud, anguished at the end.
I have the feeling I'm going to be falling over at the end of these performances, but it's worth it for such exciting music. Because of our hookup with the Bach Festival, and a gig with the Chamber Orchestra, the rest of the year is all Bach, and Brahms' Requiem in February. It will make me very, very happy to sing those things, especially my very first time singing Bach's B Minor Mass this spring.