The 68k is a very clean instruction set; you have a number of data registers, which all work the same, and address registers. There are nice instructions for math, including integer division / multiplication.
If your eventual goal is Megadrive and as you have previous C/C++ experience it doesn't seem like a stretch to go directly for the 68k.
However, there may be some difficulty in setting up a development toolchain so you can compile and run Megadrive programs, and also you would be learning the hardware features at the same time (to e.g. learn what addresses you need to poke to get something to show up on the screen). There are reverse-engineered / leaked resources for this, but not as abundant as for retro computers. When an 8/16bit console boots up and starts executing your program, it typically starts from almost nothing, on the other hand a computer typically has the screen already displaying some sensible data (like text), and has ROM operating system routines to help you.
Therefore, for the quickest, hassle-free introduction into the retro/asm programming mindset with minimal setup and immediately visible effects I'd recommend the C64 as well. For example with the VICE emulator, you can break into the built-in debugger/monitor and write & run simple asm programs directly; no toolchain setup needed. The C64 CPU instruction set is extremely limited though, you have three primary registers (A,X,Y) which all are used differently and you can forget about more complex functions like multiplication - they don't exist and must be written manually using bit-shifting arithmetic.
If you don't feel overwhelmed by the prospect of learning the hardware and having to set its state up from scratch, you'll waste less time going directly for your target platform though.