They're the top performing Arm CPUs.
They're both RISC, but P9/P10/etc is based on IBM's own Power microarchitecture (possibly open sourced / open standard), I wasn't aware it had anything to do with Arm? I could be hopelessly wrong...
Or do you mean something different with "POWER"?
And power is open in that the ISA is open. Power10 is a bang up to date CPU so the internals are all closed.