Writing a resume - traditional vs. simplified chinese
Simply depends on region. Put it this way: you use British English when you write resumes in Great Britain (and related regions/countries) and American English in the US. Nothing more than that. As you would not mix British and American English in an English resume, you would neither do so in a Chinese resume.

The places that use traditional Chinese are: Taiwan, Hong Kong, Macau.

The places that use simplified Chinese together with traditional Chinese: Singapore, Malaysia, Thailand(See this). However using traditional Chinese in such regions can't go wrong.

The place that uses simplified Chinese only: mainland China.