Hey David,
Hope all is well!
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;
Cisco Unified Messaging Feature Comparison
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6789/ps5745/ps2237/product_data_sheet0900aecd806bfc37_ps5520_Products_Data_Sheet.html
Cisco Unity Connection - with Video
http://www.cisco.com/cisco/web/solutions/small_business/products/voice_conferencing/unity_connection/index.html
There is also an "older" (like me:) doc (attached) called "Selling Cisco Unified Communications Express". It gives a feature comparison between CUE, Unity and Unity Connection.
Hope this helps!
Rob
