Learning 6502 (the C64 is actually a 6510, but basically the same thing) asm will be absolutely worthwhile, not in the sense that it is a directly usable skill

Learning 6502 (the C64 is actually a 6510, but basically the same thing) asm will be absolutely worthwhile, not in the sense that it is a directly usable skill but in the sense that it will give you a greater understanding of the lower levels of computer operation which will help in all sorts of subtle and not-so-subtle ways when you are doing virtually any type of programming. As far as ARM or x86 being more useful, I'd stay away from learning x86 (particularly modern x86) as your first assembler because it has evolved to be more of a compiler target language than a human-understandable assembler. ARM isn't as bad on this front (though it is ramping up in complexity lately). 6502, 68000 or MIPS would be my recommendations for a first-timer though.