For use in low level lighting, night mode is supposed to be better on the eyes, because it provides less glare. However, for normal every day use in normal lighting, either will actually work.
It does often come down to preference and what state the individual's eyes are in. Some will always prefer night mode, while others will want to go with normal, just because they feel it's easier on their eyes.
Consulting an eye doctor, especially if there is extensive use of a lit screen, particularly at night, is probably best. That way individuals can at least hear what may be best for their eyes.