Get a trackball and don't consider a control panel without one. You won't regret it.
In mine there is the Happs 3" trackball that comes with 3 mouse buttons prewired and simply interfaces via USB or PS/2. Easy. If you want to go for a little bling choose the red or blue transluscent and add an LED lighting kit. It will wow your guests.
There are other colors available in transluscent from Happs but you'll have to add an encoder board inorder to interface it to the PC. No big deal to go this way either. Just a little extra wiring effort.
At any rate, the mouse buttons don't interfere at all when you play games. They only work when you are using the trackball as a mouse. I use a frontend which I have disabled the trackball as a mouse in the main frontend navigation. (It is only used as mouse in Windows and as a trackball in games.) But even if you want to keep it functioning as a mouse within MAME or your frontend, once a game begins you will have no problem. The game buttons are always getting mapped to keyboard inputs via an encoder board, and your mouse buttons are not mapped to keyboard inputs so no interference. (And can't be mapped to keyboard input if they are not going to the PC via the keyboard encoder board--like an IPac, for ex). And since you can have three mouse buttons you can assign macros or other functions to your mouse buttons to enhance productivity (though I haven't bothered with it, which leaves my middle mouse button non-functional at the moment).
And the trackball games are fun. The most popular at our house are the Golden Tee golf series (very cool!) but, as mentioned elsewhere in the thread, Missile Command, Centipede, Millipede and Marble Madness are must-have classics to play in your cab. And while not "classic" per se, there are the bowling games that are often well liked by party guests because several, like World Class Bowling, support playing up to 4 players via the same interface. (Golden Tee does too.) And if you are going to have any vigourous play of Golden Tee games you may also want to consider a reinforced mount from Happ.
For me the biggest hangup I had to trackballs are the common, ugly "big black square" mounting plates. They actually don't look bad if your CP is black, but when full CP art is applied I think they are a visual distraction. But my cab manufacturer used a recessed mount so that the CP art could butt up right next to the collar of the trackball. Once they could make it look better than what you see as standard I was sold.
And another little pointer: if you plan to play the golf or bowling games don't put a joystick above it on your CP. You WILL hit them with your hands when playing. (It is common to see 4-way sticks or spinners placed above trackballs in common commercial CP layouts.)
[edited for clarity]