I would pick the existential rarity. The reason for this would be, if the coin is a truly rare coin , then I would rather one of these . A coin that there is many of but hard to find in top condition would not interest me as much. In reality these are very close to the same thing. Both are hard to find , and especially in top condition..
Existential. Preferably "known low mintage", not survival. For coins where many were minted but few known to survive, or many were minted but few known in high grade, there's always the possibility of that hidden cache being discovered and upending the statistics. See the 1903-O Morgan, or the recent cans of gold coins.
Or those mysterious business-strike 1895-P's. Or mint bags of anything from Charlotte or Dahlonega...
How about someone someday raising a few dozen 1854-S Quarter Eagles or Half Eagles in a deep sea salvage operation?
