Depends on the input device and what you're using them to play.
If you're using a device that shows up as an xbox360 pad to play modern PC games, it's nice to have the onscreen pics correspond to your actual buttons.
I have kind of a hybrid setup that uses Neo Geo colors and letters on top, with xyz on the bottom.
Most Neo Geo games only use the first three buttons anyway and they're easily remembered.
If my cab weren't neo geo themed, I'd use all xbox labels.

On some PC games, this also works with direct input controllers which uses numbers for the buttons.
I also have a moves list holder where I can swap in pics of button layouts for various games.
I can never remember the controls for sports games.
