I own an original Neo-Geo controller and the 8BitDo version, here are the differences I have noticed.
The actuation point for the 8BitDo controllers are much further from center than the original. This is caused by weaker, thinner springs which need more pressure to trigger the microswitches.
If anyone has some idea as to how to measure spring tension, I'd be glad to hear it so I can provide the original tension for modders.
Unfortunately, this is not a simple switch of one spring to the other, as the hole for springs is smaller than the old springs allow for, thus the need for custom springs.
The central stick is marginally bigger which would not be an issue if it was not for the stick cap being smoother than the original, which cannot fit onto the new stick. Thankfully, the old stick fits just fine into 8bitdo's device.
The Red, Green and Blue buttons, oddly, are marginally more washed out than the originals, which makes it all the more bizarre that the mustard-yellow button is exactly the same! There are no appreciable differences in the buttons otherwise.
The bumpers are a welcome addition to the original design, but it does baffle me as to why we didn't get a Neo Geo CD 2 controller similar to the one released for the PS3, which had an additional two buttons, making it a six button controller ideal for fighting games which also had two bumpers.
If anyone has a solution to the springs, I'd love to hear it, especially if it's available in the UK/EU. I've very willing to buy 100+ springs and sell them via ebay for others at a marginal loss for myself if that's the only option.