hi,
first of all, do you talk about:
- good old Z80 microprocessors (known as Z84 serie);
- Z180 serie microprocessors;
- modern new eZ80(r) core microprocessors;
- Zilog microcontrollers Z8Encore!(r)/XP™ serie based on Z8 core;
- Zilog eZ80Acclaim!™ microcontrollers with EMAC, PLL, RTC, PWMs, JTAG etc.

Secondary, which 51? Simplest 8051, ultra speed Cygnal 100MIPS, high integrated USB/Ethernet/etc devices?

So I do not understand what do you wish to compare. In fact, I even cannot understand why do you need to compare such different things as MCU and CPU. I have elaborated many devices where both ones are used, for example, look at this link to see one of them. This board is placed into ISA slot and controls upto 32 RS-232 devices individually (may call it "server" :)


Regards,
Oleg