I use a lot of choir samples for the classical/Russian liturgical pieces. It may sound strange but the SOV by Spectrasonics so far gave me the best performance for this type of music. It is a legacy library that still seems to be available from them in Akai format (I converted first to Gigastudio and recently to Kontakt using the Chicken Systems Translator).
I just tried Soundiron Olympus (full version) and was very disappointed with the quality of the voices. They also did a bit of false advertising for the full chorus claiming that they have "4 Multi-dynamic True Legato Vowels: Ah, Eh, Oh, Oo" (as well as soft 'mm'). That is true for the women's choir (Venus), but not for the men's choir (Mars). The men's choir has only Oo and Mm in soft version (piano). The other vowels are forte (or perhaps ff since they really yell).
So, if you are looking for Carmina Burana type of experience, you might be happy. For anything else it would be terribly overpriced. (full version is $549 at the time I'm writing this). I can only guess how pathetic the $20 version would sound...
The only possible value from Soundiron could be true legato (for Venus choir) since you can also do it in soft version as well as ability to create phrases and go beyond simply vowels. However, due to low quality of the singers in the choirs they sampled you may not be happy in all registers. For example, the sopranos lacked proper vocal technique and in the upper registers they did not have overtone mostly because they did not have their voice positioned properly (instead of resonating in the head, they were signing mostly from the throat and were a bit squeaky). They would also have a bit weird and strong vibrato in the high register which is not welcome in Renaissance or liturgical pieces. Their range was also below hi-C as far as I could tell.
I did not expect the quality to be like the Tallis Scholars, but I at least expected female singers that can reach high notes without effort and have proper pitch without vibrato. You can easily add vibrato when you need it, but you cannot get rid of it once it is sampled in.
Finally, the men's choir (Mars) did not have much range at the bottom. Apart from having those extremely loud yelling like notes that you can use a few times after which it starts sounding like some extremely loud Army choir singing military music. For liturgical music and Gregorian type pieces you need very deep steady notes in both piano and forte but not too loud dynamic. Using volume/velocity and (linear) filtering cannot dig you out of that hole.
I did not try EW Symphonic Choir. I was actually thinking of getting that one, but took Soundiron since it appeared that it had a better interface for phrase creation. However, I'm afraid the EW might also be with very limited dynamic range. It seems that most of these choirs provide cheap loud yelling voices and a bunch of useless "effects", "swells", "screams", "noises", etc., which you cannot use too much in the Choral music. For Choral music you need very rich selection of dynamics, ALL vowels (Ah/Oh/Oo/Eh/Ii/Mm), without vibrato and as large as possible vocal range (both low and high notes).
I am not aware of a single choral library that could satisfy those MINIMUM requirements. It's nice to be able to add phrases, staccato, etc. to the mix. But if the raw material is cheap, dull, and low quality, your end product would sound the same...
If you know of any library that could live to the above standards I outlined, I'd like to hear about it.