I'll let you know what I am doing and my thought pattern and you can decide for yourself:
My main concern is making the CP intuitive and user friendly since I want to be able to play with the wife and friends and I am worried about them being overwhelmed, so I think a minimalist approach is best. I decided on 7 buttons per player, arranged like this:
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The rationale is I love street fighter and neo-geo games, so I have the sic button config, and the extra 7th button is so that when I play neo geo games, you have the 4 buttons in a row. The 7th button is also good for using the "run" button on MK3. The way I see it, for MAME, there is no need to have 8 buttons, nothing that I can think of uses more than 6, and if you aren't into fighters, you can get away with 4.
As for emu's, which ones are you going to run? If you think about it, NES, SNES, Genesis and PS1 use 6 buttons or less for just about every game. Same goes for N64. That being said, personally, I think its best to play console emus with a control pad. So for my emu's I am using 2 wireless xbox360 controllers.
My advice is think about who you are designing the CP for, and what games will be mostly played on it. To me, its not worth it to stick extra buttons and controls on a CP if you are rarely going to use them. And personally, I think console games are more fun with controllers than with joysticks and wireless 360 controllers are an easy, simple, clean way to add controllers to your cabinet, and they have enough buttons and joysticks to play every console emu.