Reasons to decide for Drupal 8 instead of Backdrop CMS are (*):

• D8 is better Internationalization.
• D8 has a Symfony-like architecture.
• D8 has a more robust out of box experience.
• D8 has a much larger support community.

(*): These bullets are quotes from a comment from quicksketch (from around 'end 1Q2015'), added to his answer to the question "When should I use Backdrop CMS rather than Drupal 8?". It ends with "Maybe there should be a whole separate question for that answer" ... Hence "this" answer.

PS: There are more reasons then those above ... CMI, TWIG, etc ... Try Drupal (8) to experience them yourself.