I'm writing a school administration software package, but it strikes me that many developers will face this same issue: when communicating with users, should you use email or SMS or both, and should you treat them as fundamentally equivalent channels such that any message can get sent using any media, (with long and short forms of the message template obviously) or should different business functions be specifically tailored to each of the 3?

This question got kicked off "StackOverflow" for being overly general, so I'm hoping it's not too general for this site - the answers will no doubt be subjective but "you don't need to write a whole book to answer the question".

I'm particularly interested in people who have direct experience of having written comparable business applications.

Sub-questions:

1. Do I treat SMS as "moderately secure" and email as less secure? (I'm thinking about booking tokens for parent/teacher nights, permission slips for excursions, absence explanation notes - so high security is not a requirement for us, although medium security is)
2. Is it annoying for users to receive the same message on multiple channels?
3. Should we have a unified framework that reports on delivery or lack thereof of emails and SMS's?