Quality and reliability accuracy they're pretty close. I'd pick the px9 though mostly due to its easier to upgrade with standard ar15 parts. But the px9 doesn't hold the bolt open on the last round Wich for some people can be a deal breaker and the fx9 does hold the bolt open. Plus I've used the kak shockwave brace and the sba3 brace. The px9 comes standard with the sba3 brace and it's so much better than the kak brace. So they're both great but believe the px9 wins, atleast for me. Hope this helps and good luck in your choice.

If you Google fx9 vs px9 the first video by the last boy scout does a great comparison video and shows the pros and cons of each.

Both should be decent the honest outlaw reviews did a video on the FX9 a couple years ago, and he mentions it as being a great "bang for your buck" option all the time.
I've bought a few guns based on honest outlaws videos and hes never steered me wrong.
His original video here
https://youtu.be/JyNhPUP1ZZo

Matt, they should be fairly close in build quality and reliability Sir. That said, my personal choice would be the PSA PX9P8 gen 4. Cheers and stay safe!