There isn't really a 'preferred' layout - depends on the person. If you're into the fighting games, then a straight or slanted 6 would do it. A lot of us build cabs for the classic games that only use 2-4 buttons - and the occasional dip into 6 button games could warrant the 'odd' 6button visio template. this layout could keep the player from believing he was drowning in his sea of buttons....
Also keep in mind neo geo games used 4 in a row on a curve - at least in my mind

hence the top four buttons in that same 6button layout.
check out the controls section on the left and click on trackballs... that lists some options. nothing compares to a real arcade one, though.