Depends on the condition. The 4 extra mags with the FNP may well make the difference. 4 extra mags for the FNX would cost you about $180, so you could look at it that you are only spending $220 for the FNP.
The big difference between the two guns is an additional backstrap with the FNX, plus a difference in the safety. I had an FNX-9, and it was a great gun; very accurate once I got used to the combat sights, and the 18 round capacity was good. My FNX was very reliable and cycled everything I put in it with no problems. The only reason for getting rid of it was that I got almost as much as I spent for it (after 1,000 rounds), and replaced it with the HK I told myself I just HAD to have.
I assume the FNP you are looking at is used? What condition is it in? Why is the owner selling?