The ISA doesn't really matter (at least, not between MIPS, ARMv8, and RISC-V) when you're first diving into computer organisation. That's why one book can be sold with three different ISAs. The overarching concepts are mostly the same: you have digital logic, IEEE-754, basic assembly (add, sub, and, not, xor, load, store, shift), pipelining and hazards, caches.

Only afterwards do the differences between the ISAs come to play, when you have complex vector and ISA-specific instructions, superscalar pipelines, atomic instructions, etc.

It matters for engagement, e.g. if you can actually run examples painlessly.

The open standard ISA RISC-V has the advantage there, with open source hardware implementations (like C906 and C910) and good tooling and simulators.

RISC-V is rapidly building the strongest ecosystem.