Both these phones come with almost similar features and the user can actually opt for any one of these.
Processor: The Xiaomi Poco M3 comes with the Qualcomm Snapdragon 662 processor and 6GB of RAM whereas the Xiaomi Redmi 9 Power comes with the Qualcomm Snapdragon 662 processor and 4GB of RAM.
Camera: The Xiaomi Redmi 9 Power has a camera of 48 MP + 8 MP + 2 MP + 2 MP with f/1.79 aperture whereas the Xiaomi Poco M3 has a camera of 48 MP + 2 MP + 2 MP with f/1.79 aperture.
Check Full Comparison Xiaomi Poco M3 vs Xiaomi Redmi 9 Power
The Xiaomi Poco M3 is splashproof but it can't be said as a fully waterproof phone as there is no IP rating for the device.
And it is obvious as well for the device at its price.
Check more about waterproof phone-
Check Full Device Specifications Xiaomi Poco M3