Research interests Vs Statement of Purpose(SOP)
The intent of both types of response is similar enough to be identical, but there's perhaps a more important distinction here: this is part of a job application. The audience for this application is interested in what you have already researched, why you studied it, what you plan to research next (ideally as your PhD research), and how your past experience will aid your research plans. It's an opportunity to connect some of the material already listed on your CV, especially as CV's generally don't include much detail about the future.

This previous question ("Unclear terms: Statement of Purpose, Academic Statement and Personal Statement") touches on similar issues though it appears that the poster never received an answer they liked.