Main difference is the FNP is decocker only.
The FNX has a repositioned ambidextrous decocker/safety (repositioned, unfortunately, to right where the first knuckle of your thumb would probably like to be). The mag release and slide release are also ambi.
Also, the bore axis is reportedly lower in the FNX.
I like my FNX quite a bit, but IMO the safety/decocker lever is seriously flawed. It's uncomfortable. The gun can't be decocked with your trigger finger on the side of the slide, cuz the other side of the lever will just push on your hand. The lever is floppy plastic. The right side lever is known to shear completely off if used frequently (say, by a left-hander) or if it catches on something (since it simply hovers over the side of the gun, instead of being inset or protected in any way).