Hi Abhijit,

The feature sets are getting closer and closer with Unity and UC. This doc 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);

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

Have a look at what is coming down the pipe for both systems:)

Unified Communications Applications: Unity Connection 7.0 EFT Opportunity

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=Unified%20Communications%20Applications&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cbeeab0

Unified Communications Applications: Unity 7.0 EFT Opportunity

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=Unified%20Communications%20Applications&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cbee2ac

Hope this helps buddy!

Rob

PS: I am sure other more knowledgable NetPros will weigh in on this one :)