I come from the same lineage as the author. I did 6502 (doing C64 demos) long before I encountered the Z80. From what I remember, the Z80 offers a vastly superior programming experience. It has more registers. it has 16 bit registers. It has a shadow register set (you can switch between sets, which is handy for interrupt routines, for example) Programming assembly on the Z80 just is less of a fight.
But 6502 has 256 registers! Full ZP of them.
And the 6502 had one mouth (A) to taste and chew with, and two hands (X and Y) to move stuff in and out of the mouth with.