Is white text on dark background better for usability on smartphone / tablet keyboards than the reverse?

Which is a better and usable Smart Phone or Tablet keyboard?

1. White / light characters on Black / Dark keys on the keyboard layout.


2. Black / Dark Grey text on the white / light grey colour keys layout.

From the usability and user experience point of view, which should be the less stressful and easy on the eyes and can have a better visibility in terms of contrast and day-time or night-time usage?