Robert - it looks like there is one very important difference between BP and E3 to do with online archive access. If I am reading this correctly you need Office Pro Plus before Outlook will see online archives. Hence they are only available via OWA and there is no delegate archive access in a BP subscription as it has Office Pro, not Pro Plus.
Can you confirm?
This difference is much more likely to push us to E3 though I'm a bit annoyed at having to spend twice as much for one small feature.
Rob