What's the difference between saying '我不比她差' and '我比她不差'?
Judging from a native speaker point of view, use '我不比她差' only.
'我比她不差' doesn't make sense in Chinese, as no one uses '我比她'+ '不' + adjective.  

When you want to say 'I am better than her', say '我比她好' instead of '我比她不差'
When using '我比她' + adjective, don't use '不' in front of the adjective; native people don't understand it and don't use it like that.

When you want to say 'I am not worst than her', say '我不比她差'.

More example: When you want to say 'I am taller than her', say '我比她高' instead of '我比她不矮'