the differences are mostly in the controls, the size, shape and feel. The FNX has an egg shaped mag release and a different style safety/decock. The FNP has a round mag release and a more classic (for lack of a better term) style safety decock. The controls on the FNX are more pronounced, and stick out more, but not in a bad way (unless youre like me, see below)
The FNX is fully ambidextrous, the FNP has an ambi safety/decock and standard mag and slide controls. The FNX comes with a handful of backstraps and the FNP has a swelled one and a flat one.(edit, see below) The FNX has a higher capacity, +1 in 9mm, but the same in .40.
The FNX has a manual safety position by flicking up on the decocker. Most FNPs are decocker only, but the USG models have the up-safe position. if you specifically dont want a manual safety, go with a standard model FNP. If you do want one, you can go either way.
i bought the FNP on purpose over the FNX because i didnt like the shape of the magazine release, and the safety/decocker felt funny in a shooting grip. I went with a USG because i wanted a manual safety. See if the FNP is more comfortable for you. Sounds like the FNX is really working for you though.
Unless youre like me and just plain like the FNP better, you should go with the FNX. Its newer and they're phasing out the FNP.
Feel both. the only difference is the feel in the hands. Theres absolutely nothing wrong with either one, and theyre incredibly similar.
In a nutshell, heres what you would be "sacrificing":
fully Ambidexterous, if youre not a lefty, nothing really. If youre lefty, the choice is obvious.
+1 capacity. not a big deal, but +1 is +1.
Additional backstraps. this is up to you. maybe one of the FNX backstraps will really improve your shooting, i doubt it but its possible (edit-see below)
having the newest thing.
Unless you try the FNP and say OH WOW. compared to this i HATE the FNX mag release and safety! (like i did) go with your gut, if you like the FNX you dont need to worry about the FNP.
Edit- looking at the website, apparently the FNX only has 2 backstraps like the FNP. I could have sworn that it had about 5, thinking back to when i bought mine.