Open position

I started in a hospital last May. I was hired on as float pool, but specifically assigned to one unit. They were very short staffed but didn't have it in their budget to hire a new position. I have worked on that floor for almost 6 months now. The manager of the unit has told me repeatedly that she would be posting the position for me to bid into. Which she finally did the first of October. I was told I got the position for 11a-11p. Which is what I had wanted so she "specially posted it for me". In all actuality she posted 2 positions for that shift. the other position was filled my a gal who has been PRN on the floor and decided to come full time since losing her other job.

Now my question is...who has seniority. She has been on the unit longer but only prn. I have worked on the unit full time for 6 months now. Only one of us will be able to work the 12 hour shifts. And she's not willing to comprimise.