Some of it might be how it is haphazardly labeled by some sellers out there on Amazon, but to be exact, 30 mm are going to be the diameter size for larger Japanese style buttons, and 28mm is going to be American style. Japanese also come in 24mm.
Japanese and American buttons feel and play different, so if you are not familiar with the differences, go with American. American are generally going to have a cherry switches that clip onto the bottom, rather than built in to the button itself. Japanese buttons are often desired for people into Japanese fighter games and Shmups (shooters).
A 1 1/8" hole drills about 28.6mm, is a really nice hole size for American buttons. Tolerances are not exact from button to button, so it generally always works. I recommend it for an all purpose bit if you are sticking with American buttons.