It is really futile, to compare the Z80 Micro Processor and the 8051
Micro Controller. This is like comparing oranges to bananas.

The Z80 had a basic 8080 instruction set with Z80 derived extentions.
As an 8 bit processor, it was an extremly capable unit.

The 8051 Micro Controller and its derivatives where never intended to do the same job as was the Z80 or any other Processor. (note the differenciation between Processor and Controller)

Regards, Horst Schmidt