Either you can go for extension of the standard service, or have your own custom service and consume the same in your UI5 application.
Extension would be helpful, performance point of view.
You might want to check out documents here :
http://scn.sap.com/docs/DOC-53177
- Extending a Fiori App - Simple Use case - Part 1by Murali Shanmugham
- Extending a Fiori App - Simple Use case - Part 2by Murali Shanmugham
- Extending a Fiori App - Simple Use case - Part 3by Murali Shanmugham
- Fiori Extension - Reuse OData/UI components - Part 1 by Murali Shanmugham
- Fiori Extension - Reuse OData/UI components - Part 2 by Murali Shanmugham
Regards,
Tejas