Ok, some of these things (dual alarm) are new with 4.2. Also new in 4.x, I think, is that if you choose the default Gmail account sync in the iPhone you get an option to sync your calendar. I think that used to require some separate setup. Note "Exchange" means ActiveSync.
My impression is that Apple likes CalDAV/IMAP and Google likes ActiveSync; they call it GoogleSync.
- CalDAV: get two alarms, I think color assignments might be coordinated across accounts? Multiple calendar configuration requires a secret web page accessed from any browser.
- Exchange: can invite people and can sync more calendar types (holidays,etc). Multiple calendar configuration requires a web page accessible only from the phone
- IMAP: can choose between delete and archive, more iPhone like UI.
- Exchange: Push works much better, can't delete email, only archive
- IMAP only - not sure this is worth anything anyway. Use Simplenote.
- Exchange only