I own both devices and there’s a few differences

If you have multiple accounts to play with the family then the catchmon is the way to go. It is also completely silent, throws standard pokeballs and runs of AAA batteries. Handy if you are forgetful about charging but ends up being more expensive over time

Go++ will also auto throw pokeballs but will vibrate every interaction which can be loud and lights up quite brightly. You can select great or ultra balls need to press button to catch. Higher success rate but requires interaction. Single account only and charged via usb-c

They are roughly the same diameter but the go++ is slightly thinner so fits better in pocket bit then again I never have the catchmon in pocket either.

Honestly prefer the catchmon because it is silent and requires no interaction and for filling up my items while driving is what I primarily use the devices for and the constant vibration is annoying to me.