what difference is between Z8 encore MCU and 8051 based MCU?
oodles and bunches.

if there is big difference to study microcontoller by using 8051 based MCU and Z8 encore MCU?
lots, however, after a while we all end up using different controllers, so maybe - just maybe - it does not make much difference which you start with.

One note: however, if you do not go with the '51 you will not get any help here.

Erik