Hi Paul,
Major changes between the versions are more feature driven but the operation and look remain basically the same.
Feature Changes:
Extension Mobility Cross Cluster
Enhanced Cisco Agent Desktop extension mobility support to include telephone devices registered across a CUCM cluster.
Support for CAD to XMPP Gateway
CAD and CSD can communicate with the new XMPP-based Cisco Unified Personal Communicator via the CUP XMPP Gateway
Independent Client Configuration Package for Automated Deployment
Agent, Supervisor, and Desktop Administrator installation files (MSI) are available independent of the server installation. These are configured using an MSI utility that can be executed from a windows PC
You can find the documentatation for the agent desktop here:
http://www.cisco.com/en/US/docs/voice_ip_comm/cust_contact/contact_center/crs/express_8_0/user/guide/cad80ccxug-cm.pdf
For your reference the CAD 5.0 documentation can be found here:
http://www.cisco.com/en/US/docs/voice_ip_comm/cust_contact/contact_center/crs/express_5_0/user/cad/cad64ug.pdf
Hopefully this helps you make your decision.