PPC is dead from a consumer perspective. While there's nothing stopping it from being a hobbyist platform, it's expensive as hell and there's not a lot of hardware to choose from.
PPC Macs are cheap and powerful
X86 is well-supported and cheap, though there's a zillion different hardware configs. Getting it to run on everyone's PC's can be tricky because of this.
ARM is cheap as hell, and there's plenty of real cheap hardware available (the omnipresent Pi and boards like it). The hardware is a bit more uniform, but performance is lower than X86. Still, it's cheap as hell.
From those two x86 (x64) is more powerful. So if performance is your goal you choose x86 platform. If you want to go mobile then ARM. Or both. Dump idea of having native code and compile everything to 68k target and have 68k emulator built-in.
I'd prefer a Pi port, but that's subjective.
Actually RPi is one of the worst ARM offerings. There are cheaper and faster ARM based boards available but they are less famous and people always think there is only RPi...
