Fast memory is within 24bit memory space (the only space for 68000, 68010 and 68ec020). The area 0x200000 -> 0xa00000 is reserved for the memory in the memory map, thus maximum 8MB.
Z3 memory is mapped to 32bit memory area and thus is not limited in this way.
For WinUAE there is no difference except that the address of the memory (and that 68000, 68010 and 68ec020 can't have Z3 mem). Since it's emulation, both memory is equally fast. However, some rare apps might require memory at 0x200000 onwards, and with these you need to add some "Fast Memory", even if you have "Z3 Memory" set already.
