Hello Percival,
The Z80 is a great processor. It is still used in lots of applications. I think most of the credit reading merchant accounts use a Z80 inside of them.
If you base the Z80 Vs the 8051 based upon past history the Z80 would beat the 8051 hands down. It was used in the CPM computers by tens of millions. I think it's software instructions set is probably better than the 8051 also.
My two cents, I am sure many will disagree.
Regards,
Charles Bannister