Also they aren't as different as a motorcycle and a car. It's more like compression ignition vs spark ignition.
I tried to keep it relatively simple. They have different use cases like cars vs motorcycles, and those use cases tend to lead to different focuses. We can compare in multiple ways:
X86 like motorcycle:
more torque (higher clock speeds, better IPC)
single or dual rider - fewer, faster cores
less complicated (less stuff on the SOC), but more intricate (more pipelining)
ARM like motorcycle:
simpler engine - less pipelining, smaller area, less complex cooling
simpler accessories - the engine is a SOC, but you can attach a sidecar (coprocessor) or trailer, but your options are pretty limited (unlike x86 where a lot of stuff is still outside the CPU, but that's changing)
The engines (microarch) aren't that different, but they target different types of customers. You could throw a big motorcycle engine into a car, and maybe put a small car engine into a motorcycle, but it's not going to work as well. So the form factor (ISA) is the main difference here.
But yeah, diesel vs gasoline is also a descent example, but that kind of begs the question as to where RISC-V fits in (in my example, it would be a diy engine kit, where it can scale from motorcycles to cars to trucks to ships, if you pick the right pieces).