If you basically don't know how to code in assembly, learn 16-bit x86 with whichever method you find, it won't be wasted. You can extend most of the knowledge from 16 to 32 bit substituting register names ax,bx,cx,dx,si,di,bp,sp y eax,ebx,ecx,edx,esi,edi,ebp,esp. You can extend that to 64 bit with rax,rbx,rcx,rdx,rsi,rdi,rbp,rsp. Learning 16-bit x86 will have you learn about segment registers, but it isn't wasted either, protected mode consists of quite a lot of complications on segment registers, virtual memory addresses (which you can mostly ignore as an applications programmer as compared to a kernel or driver programmer), and interruptions.
I wrote a book to learn x86 programming and it was published in 1994 back before "real digital age", oriented to be the best didactical text possible (I had to learn the hard way, I wrote what I would have had to read 10 years earlier). Over 20 reprints, recommended in all Spanish-speaking universities (yeah, I know, if I had known then what I know now I'd have done it in English). It was discontinued 10 years ago or so, they asked me to revise it for the modern world, and it didn't really make sense: rewriting it for what assembly language is and is used for in today's world would be a ton of work because it's a qualitative difference, and if I added an extra chapter explaining 32-bit and 64-bit changes and letting the publisher stamp a "2010 edition" logo on the cover would just be scamming people, which I don't want to do. Here is the link to a scanned copy of the original: https://www.dropbox.com/s/sz6rinfhyc8sai6/Lenguaje%20Ensambl... . Could prove useful if you can speak Spanish.
Honestly: I learned Z80 assembly first, in the 80s, and then switched to x86 very easily. Learn whatever assembly language first, what's hard is learning about registers, flags, memory... and if you learn it will, then you can switch to another architecture quite easily.