Well, one thing that Power 9 would give us that X64 won't do as well is run our current software.
Even if both platforms were running QEMU, a Power 9 platform would run SAM460 or X5000 emulation better the X64 systems
Its simply a matter of lower translation overhead.
Power 9's instruction set is related to PPC, and X64 is completely different.

Raptor's board IS a bit overpriced, but if a larger than ATX, 2 socket board can be built for only a few hundred more than an X5000, couldn't a 1 socket ATX board be built for less?

And, yes, the G5 could be used as a starting point for 32 or 64 bit MorphOS.

So while I understand the X64 argument, moving forward with Power would be more evolutionary.