I cannot reach the upper notes of O Salutaris (F5 and G5), and I have five days to practice.
In this case you might want to avoid picking that piece, especially if you hope to be in the soprano section. (Similarly, if you want to be in tbe alto section, you should try to find a piece that shows off the lower part of your voice.)
What factors should I consider in making my decision?
Read Andy Bonner's answer. Then think about the factors the conductor will be considering:
- can you sing in tune?
- do you have a good full sound without signs of straining? Is that sound in tune?
- can you sing quietly? Is the tone breathy or clear? Is it in tune?
- do you pronounce words clearly? Do certain vowels cause you to sing with poorer tone or poorer tuning?
- which section should I put you in?
In other words, the most likely decision process is first, can the singer sing in tune, and second, which part should the singer sing. The second question is lass critical, as you can always move from one section to another.
The most important thing is to pick something you're comfortable singing. Your aim is to acquaint the decision maker with your voice at its best. Don't try to sing something impressive unless you are confident that you can sing it impressively.
If you have trouble with part of your range, it will come out during the part of the audition where you are asked to sing some vocal exercise. There's no need to pick a piece that exposes it.
If I were auditioning singers for a choir of this description and you came in with some prepared solfeggio of a piano piece, I'd probably be favorably impressed. If you don't have any songs ready, I would suggest learning something fairly simple whose melody you already know.
If you know O Salutaris fairly well, consider singing it in a different key. There is a version in C major for low voice (are you sure you're not an alto?).
The Pie Jesu is very difficult to sing well. It's easy to know what notes to sing, but sustaining the long slow phrases demands a lot of breath control. This is also true of O Salutaris, to a lesser extent. I would suggest looking at songs with a medium tempo, not too slow, not too fast.