Happy Lumin X1 user with the EtherRegen fiber connection. My own experience also concluded that just fiber is not enough although it advanced the quality a lot. Adding the network clocking and clean circuit of the ER really does bring out the best of the X1 capabilities. I will say it is a must have for X1 owners. Caveat being that the results depend on the condition of your network. Some are better at it than others.
Coming to the P1, I too am curios how it matches up with the X1 in sound quality. I chose to move away from USB audio over to network streaming as in the case of the X1. The tradeoff being the limitation in music source but the positive being that the quality is outstanding with what is available, and with a lot less headaches. I am a tweaker and have tinkered a lot with PC audio and I love the X1 setup. To be fair, I still have a Roon server in the mix but I do not let it to get too close with my X1 (jk).
The only twinge I have felt lacking in the setup is when it comes to Amazon Music HD. There is just no way to get lossless Amazon into the X1 as of yet. But the P1 can. It is not making me think of changing anything though, as I am in audio heaven with the X1
To the OP, I think either will be just fine. Good luck with your search and Thanks for asking your question.