At a little over melt, the $20 gold coins must have a better future, but I'm seldom good at picking the coins that go up in price on a short term basis.
None. If a coin was undervalued in the market then demand would drive its price up. There are many coins which one would think should be worth more based on their rarity but have few interested buyers. There are also many coins which based on availability would appear to be overpriced but aren't simply because of the number of people who want them.