If you look at the mintage, you come to the clear conclusion that the 1915-S is likely to be a better Buffalo nickel. The fact is, the 1915-S is a better date, although there is some question as to whether it really is as good as you might originally assume, based on its mintage.
Certainly the 1915-S was the low- mintage Buffalo nickel of the year with a mintage of 1,505,000. Except for the 1913-S with the buffalo standing on a line, that made the 1915-S the lowest mintage up to that time and the 1913-S line type was only a partial year?s mintage. If you added the 1913-S with the buffalo on a mound, the San Francisco mintage in 1913 was much higher than 1915. Those points, however, would probably not have impacted the collectors of 1915. Whatever the totals, they would still have been high compared to other issues, so there was not going to be much, if any, saving of the 1915-S.
There is reason to question whether there was going to be much saving of the l915-S under any circumstances, as we do not have a good feel as to how many would have been collecting nickels by date and mint at the time.
Nickels had only been made at mints other than Philadelphia since 1912. Prior to that, the only nickels made were limited to production at Philadelphia. With no holders or albums at the time, it is reasonable to question how many were attempting Buffalo nickels by date and mint, and how many were still assembling just date sets, which had been the historical way to collect nickels.
We do know that if a 1915-S got into circulation, it would not have a good survival rate because the date was too high off the shoulder. It did not take much wear for the date to start to disappear. There was a compound introduced to restore dates, but it caused damage, so a Buffalo nickel with a restored date will never even be a G-4.
In the case of the 1915-S, it does not appear that it was a date where enormous numbers were lost because their dates disappeared.
That said, the current $40 price in G-4 does suggest that a lot of 1915-S Buffalo nickels might well have lost their dates, vastly reducing the supply today in lower circulated grades.
In the case of other grades, the prices we see today make a very strong case for the idea that a 1915-S was either saved immediately or not for many years. That is simply because the 1915-S, like a number of other Buffalo nickels of the period, is a tough coin to find in a grade like XF-40, as few new collectors came along to save Buffaloes in the 1920s.
In the case of Mint State examples, it is not surprising that there are relatively few. That produces prices of $625 in MS-60, which is high for a date of the period, and $3,400, which while high, is not as high as some dates, especially from the 1920s.
At the Numismatic Guaranty Corporation, which has seen hundreds of examples of the 1915-S in Mint State, the total in MS-65 or better is just 68 coins. Professional Coin Grading Service has seen 113 at MS-65 or more, and neither service reported a 1915-S in a grade better than MS-66.
Certainly, the numbers are low enough that if there is any significant increase in Buffalo nickels in top grade, the 1915-S could easily prove to be a date where there is simply not a large enough supply to meet new demand, at least at the current price levels.