Yep demo coins are exactly that, coins that were used to initially demonstrate the concept of hologrammed coins. ie public and private key under the hologram, and the hologram peeled to show its tamper resistant function. All the demo coin addresses appear in the casascius full list text file, but have never had any btc deposited to the address. All are 2011 1 BTC brass coins. Most that were done were 2011 series 1 error holograms. I have only seen 1 demo coin with a 2011 series 2 hologram. Not sure on actual mintages of either, but low. I keep meaning to get an auction going for a series 1 demo I have. Will sort that in the not too distant future

Void coins are based on the brass halfs. As press / media were getting more interested Mike used these to show the concept again, and be a bit more photogenic, by not having a peeled holo. Again unsure on mintages, but lower than the demo. I recall EODguy did a bit of research on these. I seem to recall he tracked down around 7 ish ... he could probably correct me. You seldom see these for sale.