David Akers (1975/88): The 1878-S is most definitely not a common date. It is not particularly scarce in circulated grades but in strictly uncirculated condition it is very difficult to find. Overall, the 1878-S is more rare than the 1875-S and 1877-S and on a par with the 1874-S. It is far rarer than the common 1876-S. This date is most often available in EF or lower grades although AU's are also seen fairly often. In average uncirculated condition, the 1878-S is very scarce and in choice or gem mint state it is decidedly rare. In my opinion, this issue, as well as the 1879-S, 1880-S, 1881-S, and 1882-S, are all very underrated in uncirculated condition.
The Saddle Ridge Hoard of gold coins, discovered in northern California in 2013, contained thirty-four 1878-S $20s, of which twenty-seven were gradeable, four were Mint State, and one was tied for Finest Known.
Saddle Ridge Hoard
Heritage 9/2006:4139, $23,000
RNB Collection - Heritage 1/2007:3733, $18,400