The feature sets are getting closer and closer with Unity and UC. The doc below nicely encapsulates some of the differences. UC is not integrated with Exchange but can currently support up to 7,500 users (and this number is increasing - see below) Also, UC 7.0 supports Active-Active redundancy which was a stumbling block for many enterprise type deployments that require full redundancy. Another rather large difference is that Unity is a Windows based model and UC is a Linux based appliance like the newer 5.x, 6.x and 7.x Communication manager versions.
For "full" Unified Messaging you will need Unity, for "Integrated" Messaging via IMAP access then UC 7.x is a great choice;
The differences are nicley encapsulated in these docs;
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.