You are asking something similar to:
"Should I buy a bicycle or a Car?"
Each device has a specific use. The answer depends on what you really need.
In my case, I have always been terrible with watches; these simply don't last in my wrist. The only watches that have lasted me a very long time are Casio G-Shock; the rest, end up breaking up.
While the Apple Watch may be something like a gimmick (It takes no effort to take your iPhone out of your pocket and manipulate it with just one hand, rather than using both hands to use your Apple Watch).
Also, it's one more device that you will need to keep charging or it will run out of battery and become useless.
On the other hand, a Mac Mini could be used for so many things, even to make money with it.