OpenPOWER has no advantages over RISC-V. The performance is going to be the same given the same level of investment into the silicon design. These are just ISAs, and a lot more goes into CPU design to make them fast.
It's possible that OpenPOWER has no advantages today, but I remain curious how we got here; my understanding is that POWER and SPARC both had open source versions before RISC-V was created, so why didn't those get picked up but RISC-V did?
RV32I/RV64I are very very small ISAs. RV64G is pretty small -- comparable to, say, MC68010, and a lot simpler than i386.
These small subsets will be supported by the ecosystem forever, for those who want to use them. They will also remain, unchanged, at the heart of more complex standards such as RVA23 and successors, and make up most of the instructions in programs.
RISC-V's success has been mostly in small embedded systems — where a POWER chip would be overkill or unsuitable. The core ISA is very small, thus cheap, and then extensible. The design even encourages proprietary extensions.
Then it has grown from there as ISA extensions have been ratified.
Depends on your market. If you want a high end server/workstation class processor then openPOWER absolutely has an advantage because they have a shipping product that is (somewhat) competitive in that arena and despite the risc-v promises the products look like early 2010 era arm server machines. OpenPOWER have 30+ years of instruction set and architectural tuning to fall back on, and that is nothing to sneeze at if you actually want good vector perf, a stable toolchain, or a roadmap that doesn't depend on VC's staying in the game.
On the other hand if you looking for a small embedded one off solution then sure, risc-v. But unless your already sold on risc-v, there are a lot of other competitors in that space which have such a huge breadth of product offerings that it might be hard to justify a risc-v solution solely based on the processor architecture.