Given that money is, unfortunately, a prime driver in any procurement process, I would suggest that a turbo-prop is the way ahead. Significantly lower fuel consumption, and having seen the number of support required for jet and prop MPA, prop again wins. When looking at the P3/Nimrod, there is not a lot of difference in top speed and the P3 wins on endurance, and probably range. What sensors you put in the aircraft is another matter altogether.
It is easier to compare similar generation aircraft as in the Nimrod MR2 and P3, however more difficult if you try and compare a P3 against a P8 or Nimord MRA4. You would expect the newer generation aircraft to be much cheaper to operate, and hence better value for money. An educated guess is that the fuel consumption of the P8/737 is comprable to a P3, which is significantly less than the Nimrod MR2. How this compares to a Nimrod MRA4 I cannot say, suffice to say that the MRA4 will burn more fuel.
Y_G