Hello,
I encountered a similar issue to yours.
In my opinion, the P3 is primarily used as a preamp, while the GR40 serves as a DAC and preamp.
However, I ultimately opted for the P3 over the GR40 due to the RCA and Bluetooth input and output ports, which are the most essential features for my needs. (Although, you can still use an adapter to connect to coaxial cables.)
If you typically use optical, Bluetooth, and USB connections, the GR40 would be a more suitable choice.