68000 is a really nice chip if you want to get into retro computing. The assembly language is considerably easier to learn than Z80, as the instruction set is far more orthogonal, so there are far fewer special instructions and modes that only work with specific registers. There are also C compilers and other languages like Forth readily available, so you can set up a PC hosted cross-development toolchain exclusively using FOSS software.
8051 is the classic MCU for embedded systems. Derivatives are still being designed in to commercial products 35 years later. Due to its historical massive popularity, and long lifespan, there are vast numbers of academic resources for it, and again the toolchains for cross-development are readily available.
Either would be a far better choice for a relative beginner than an obscure Z80 derivative and a pre-build board lets you 'hit the ground running' and get your feet wet designing add-on boards for various purposes before you tackle the major project of designing your own processor board and writing a monitor program for oit from the ground up.
One caution however: If either of the boards has logic in a PAL chip that is no longer available, and has no compatible replacement, or if the source 'code' for the PAL is no longer available, you would have to be incredibly careful NOT to make any mistakes interfacing to it. You should prioritise writing a program to dump any ROMs over a serial link to your PC, and get a spare one programmed and tested as soon as possible, certainly before you do any hardware interfacing. All the other chips should be reasonably generic or at least findable as N.O.S, so can be replaced if you blow something up.