To start with, there was a change in GTS Plugin with 7.2 making it more independent.
There are a huge number of changes in SPL like TREX, Screening logic and functionality enhancements, new UI for SPL, zero footprint case management etc
Support for ITAR, US Re-Export, and a number of other compliance enhancements like checks on inbound deliveries.
In Customs Management, support for latest regulations like FDA, ISF, etc ; a number of additional certifications; TM integration, support for special customs procedures like IPR etc.
The above list is not exhaustive and just subset of all added things.
As Jasmit said, you can refer the delta guides at SAP Site for complete details.
Your best ROI for going with GTS 10.0 will be if you are looking to provide a more convenient user-experience for the end users . The NWBC is a good add-on.
Compliance, customs has a good deal of change. So eventually it all depends on what functionality is already implemented in your organization and the modules you would like to optimize.
Let me know if you need more info.. you can also refer to the delta guide that is available at SAP site for the new features in 8.0 and 10.0 .