ITIL&ISO 20000 over ISO 9001 in the IT area

If a company implemented ISO 9001 - that's excellent foundation to add other management systems, like Service Management System (SMS) based on ISO 20000-1.

SMS (through required processes) goes deep in daily activities of the IT Service Management team, it covers whole lifecycle of the IT service(s) and requires good governance practicies. That's IT service specifics (although it could be applied to almost any kind of the service). Additionally to the processes ISO 20000 requires management system to be established. And that's where QMS (if already implemented) has many common elements with ISO 20000.

So, if there is ISO 9001 already in place, ISO 20000 implementation will enable better management of IT services as well as better integration in overall business activities of the company.