In the world of numismatic coins, aka rare gold coins, value is partially determined by rarity. Rarity is typically described by number of coins known to exist in each grade. It is where rarity and quality intersect that determines value. Therefore, the higher the grade and rarity the higher the value; the lower the grade and rarity the lower the value.
There exists a segment of the numismatic gold coin market called “common dates” or “generics.” These coins are the easiest to obtain and are of the lowest rarity inside of each grade. For example, a common date coin in the $20 Liberty type would be the1900 Philadelphia mint (1900P). An example of a common date coin in the $20 Saint Gaudens would be the 1908P no motto.
Common date coins tend to react more to fluctuations in the spot price of gold, especially in lower grades. Although they do tend to be slightly more stable as they do not fluctuate dollar for dollar. They will tend to move when spot makes larger more pronounced moves. Bullion fans tend to like these coins because they have the bullion content like American Eagles do, but also have a small numismatic premium that blends the two worlds together. This is especially true as you get lower on the grading scale.
The VF (very fine) graded $20 Liberties sell for a couple hundred dollars more than what an American Gold Eagle sells for (bullion + numismatic premium). Obviously the higher the grade is the further away from the spot price a coin will be, so an MS64 graded $20 Liberty common date sells for just under $3,000. Therefore, if you are looking for a nice coin that tends to perform more like bullion but tends to be slightly more stable, a VF $20 Liberty common date might be for you.
Performance wise it makes sense to obtain coins in the mint state range because they do tend to perform better over the long run than bullion does. For example, gold bullion has averaged 18.37% per year over the last ten years while the $20 1904P MS63 has averaged 23.53% per year, which is over 5% per year better. Someone who is looking for the best performance over the long-term would be more inclined to acquire “better dates” in higher grades as historically they have performed much better than common dates and bullion.