So, any feature of a processor is a reason to say something is better than the other?

Ok... The 6502 does not have port-I/O. Everything is memory mapped. Therefore, the Z80 is better.

Or, the Z80 has more registers than the 6502, therefore the Z80 is better.

Or, the Z80 can do limited 16-bit operations that the 6502 can't, therefore the Z80 is better.