If artwork is a high priority for you, then swappable is probably better for you.
Artwork is NOT what gives me the "arcade experience" though.
MY arcade experience comes from not having a bunch of useless controls cluttering up the CP for the game I am CURRENTLY playing.
There is no way around that with swappable panels--except to make ALOT of swappable panels.
I personally believe that the do-all control panel takes away much more of the experience than artwork.
If you think back to the REAL arcades, it was not uncommon to see Street Fighter in a painted over Defender cab with very little real ARTWORK.
I NEVER saw ANY arcade game that had a trackball, a spinner, TWO Tron sticks, and a 1/2 dozen buttons on it though--regardless of how pretty they made the artwork.
Playability is my primary objective.
If I have Robotron going, I want TWO 8-way Wico sticks--just like it had in the arcade.
If I have Discs of Tron going, I want an up/down spinner, and a TRON stick.
If I have Donkey Kong going, I want a 4-way and A button.
THAT is how everything WAS at the arcade.
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The big PRO to modular that you overlooked is that you don't need to buy multiple controls to make multiple panels.
Spinners and Tron/Xenophobe sticks are a good examples of this.
You need one Tron stick, with a spinner, for Tron.
Now the decisions/trade-offs start.
Do you put on TWO trigger sticks, or FOUR?
If you only put on two, you eliminate ALL the two-player tank games.
If you DO put on FOUR, and the spinner for Tron, do you put 4-buttons on there as well, or buy another spinner for playing Blasteroids/Star Trek, etc....
If you DO put the buttons on that panel, you eliminate 2-P Blasteroids as an option.
If you opt for another CP for Blasteroids, do you buy another Tron stick for this CP, or another spinner to put on the "tank" CP?
Trackballs are the same way.
Do you only put on one, and forget about 2-P Marble Madness, or do you put on two?
Do you add 3 buttons to that CP for Missle Command/Centipede, etc...?
How about a 4-way to play Wacko?