I'm familiar with Unity, but not with Unity Connection. We just moved over to CUWL licenses and have a license for Unity connection now. So what is the difference between it and full blown unity? Does it have all the same features?
Unity is intended for UM, message store is corporate email (Exchange or Domino), Windows 2003 server and is a member server of the domain
Unity Connection is VMO or INTEGRATED Messaging (i.e., IMAP). Message store is internal, Linux platform with Informix DB just like CUCM, CUPS, etc. Can do DirSync but is not dependent upon AD/Exchange, etc.
If your client wants UM, they need Unity. If they want VMO or Integrated Messaging, Unity Connection is the likely answer.