if you're interested in 6502, then I'd go straight for 6502 - the Z80 is quite different, and while some knowledge may be transferable, much will not and you'll just be learning things you don't need now.
For example:
Z80 has lots of registers, relocatable stack.
6502 has lots of addressing modes, Zero page, fixed stack.
If you had never done any programming before, and may struggle with things like variables, arrays, mathematics, I'd maybe suggest you tried Basic, Java or C first... but I see from your other post you've done programming before - so It's not relevant to you.
You mentioned you've done 68000, and maybe you'd like to do it again... in that case, If you were really struggling with starting with 6502, then 68000 is easier, due to the extra registers and functionality - so starting on 68000 and moving to 6502 would make more sense than Z80->6502 for you.
Of course I've got lots of tutorials on 6502 I hope will help you, but it's worth checking out 'Easy6502' as well... it's web based, and may help if you're looking for a way to get started with the language easily as it's web based
https://skilldrick.github.io/easy6502/
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Interested in CPU's :Z80,6502,68000,6809,ARM,8086,RISC-V
Learning: 65816,ARM,8086,6809