They are very similar.... FNP came first. FNX has some revisions. This particular FNP 45 Tactical is the most accurate pistol I have ever shot.
From FN:
"The FNX-45 will look and feel very similar to the FNP-45. Most of the design enhancements were mechanical for improved reliability and endurance. The FNX-45 will come with 3 magazines (new design and not compatible with the FNP), all 3 will be 15 round like the current Tactical and Competition. The pricing should be similar to the FNP-45."