I own both the P99 and the P30, and I am happy that H&K copied so much from the P99, which came out 14 years ago.
The P99 grip was designed by Morini, an Italian Olympic pistol grip designer, and when you pick up a P99, you can tell as it was the most comfortable pistol grip I'd held at the time. H&K also outsourced the grip design by hiring Karl Nill (Nill Grips) to design the grip for the P30, and it too has a very good grip, it is the only pistol I have held that feels more comfortable that the P99.
The P99 was the first pistol to use replaceable backstraps, that almost every other polymer pistol has copied. The P30 was the first polymer pistol to take this one step further and put replaceable side panels as well, to give you I think 27 grip possibilities in the same pistol.
The slide of the P99 angles inward as it gets closer to the sights, just like the P30, and I've heard that this is to help put the sights in focus as you are bringing up the pistol.
Both use DA/SA triggers, and both carry 15rd mags. If anything, the P30 is the "wannabe" of the P99, which is why I like it, and why I bought it
, the P99 was an innovation when it came out, and it provided lots of new features in a pistol that everyone else has seemed to want to copy.
If the FNX had worked out for me, I probably would have never owned a P30, since I was looking for good quality 9mm pistol that could be carried cocked & locked. The FNX shot very accurately in my hands, and it has the best combat sights I have ever seen on a pistol, but mine had lots of little issues that screwed up the experience and made the pistol feel like a low quality product. FN will not replace an FNX pistol that has loose backstraps which moved and changed my grip while shooting, or mag insertion issues where you have to push the base of the mag forward to hear it click into place, which is hard enough while the mag is empty, not to mention when the mag is full with 17 shots and it feels like the follower is hitting the baseplate while inserting it. There were other issues as well, but these seem to be the most common. If you do choose the FNX, make sure to check for these issues BEFORE you buy it, I was one of the first people to buy this pistol so nobody warned me.
Personally, I'd choose the P99.