I have always felt a phrase bar to be a musical sentence. With that thought in mind, that would be where the melody is being played. That would explain way the flute would have the phrase bar playing the melody and not the violins who are only playing accompaniment.
The slur, as was stated, makes the notes played together without a breath or in choral without a breath.
Clarity is still needed to know if both can be used. I would think so but I am not an expert.