Do I stay or do I go?

I've been fairly blah about life right now, especially my job, so when there was an opportunity to apply for a job in a different department that was interesting to me I jumped at the chance and somewhat impulsively, submitted my resume. This is a job that I had previously applied for and did not get but that I really wanted at the time and many people have told me since that I would be perfect for. I was talking to the people that work in that department and they told me that I should apply for the job and that I would be perfect. I had an interview with the manager yesterday morning and the department this morning and the Director later this afternoon. All signs point to me being offered the job. I should be excited, yes? Here's the flip side of it all....

I told my current manager that I applied for the new job and she was not happy....at all. I told her my reasons for it and what my long term plans are (law school) and how I wasn't happy in my current job. I felt unappreciated, undervalued, and underutilized. I was and am a wasted talent. (i know I'm sort of tooting my own horn here, but that's how it goes). She sat and listened to me and then when I was done talking she said that she completely agreed with me. She then proceeded to say how much she doesn't want me to change departments and talked through the whole thing with me. Talked about the problems I saw and what needed to be done. It ended with her telling me to write up a job description and she would go over it with me and adjust it as needed and approve it. She basically told me I can make up my own job within the department. I told her that if the new job offered more money, I would probably take the job. She said ok, but if I chose to stay she would go to bat to get me more money in June. It couldn't be sooner because I just got a raise a couple of months ago.

So now, I'm not sure what to do. Do I take the new job if it's offered me, which is highly probable (although, now that I said that I probably won't get the job). Do I stay where I am and see if I can make is something exciting? I have no idea what to do. What do I do????