On the Artic Mini Mame I opted to have NO admin buttons at all.
I used the time honored method of mapping the player 2 start button to both "Coin 1" and player 2 start, which means I don't need a coin button. It is pretty simple, press player 2 start once to insert a credit, and then you can press player 1 start for a one player game, or player 2 start for two players. So far no one seems to have a problem with it. I used a shift key (down and player 1 start) to exit the game. I actually don't like people to work the menus on their own. I tend to just load Galaga, Dig Dug, or one of the Pac-games if people are coming over.
On my Amazing Mame I am using a "Power Point remote" for admin purposes. It can pause, insert credits, and exit the game (i basically only use it for mouse functions and to exit). I also have the actual coin mechs wired, and have pause and tab buttons located inside the coin door.
If you let people pick their own games then they will pick something completely stupid, say "This sucks" and then lose interest. Sure, knowledgeable gamers will load up City Connection, Robotron or Street Fighter 2, but the average person will end up playing Logger, Mikie or Wall Street.
That is why I picked a non-intuitive way to exit the games on both machines. In fact, I doubt ANYONE would ever figure out that in order to exit game on the Amazing Mame that you have to pick up a remote control off a nearbye shelf and press the left button!
Basically, I don't trust my friends to know what to pick at first. Once they get used to a few games then they start asking for them, and then I load those up. Current favorite title on the Artic Mini seems to be Junior Pac-Man (Speed up), and favorite on the Amazing Maze seems to be Money Puzzle Exchanger.