FNP or MBA?

I am a Rn nurse with 15yrs. experience and make pretty good money (42.84/hr). I am interested in going back to school and if I go to the university that I work for, the tuition will be paid for. I can't decide whether to go back and get a MBA or become a FNP. I would go part-time and finish in 4-5 yrs. instead of 2 yrs.
Here is the thing. What kind of job would I be able to get with an MBA. I have been an Rn for 15yrs. with virtually no business experience. Plus it seems like everyone has an MBA now, a dime a dozen. However, can I make MORE money with an MBA vs. a FNP??
By the time that I finish with a FNP program, my base pay as a RN will be around 95-98K WITH good benefits. Plus, if I stay 15 more yrs, I can retire with a pension immediately and I will only be 54yrs. old. I need to stay at my current Rn job at least 20yrs to get a pension (albeit much less) and won't be able to collect it until I am 54.
It seems as though FNP's only make around 100K, so I am thinking why should I go back to school just to make what I am already making?? Honestly, I think all NP's should be making a minimum of 125K/yr. Anything less is cutting yourself short. I do feel that getting a FNP will allow me to move to different parts of the country and make good money (esp since I want to move from Chicago to Phoenix in the future).
I just don't know which route to take? Any advice?

James