Thorough research in city directories, one quick example

I recently had occasion to compare physical and virtual city directory holdings. The results for this one location -- Decatur, Macon County, Illinois -- are as follows.

Assume that you're researching families that might be in Decatur over a 50-year period. Which source has the best collection of city directories 1870-1924?

Neither.

Exclusive holdings for each are in bold underline.

Ancestry has 1889, 1893, 1896, 1899, 1903, 1907, 1909, 1910, 1911, 1921, 1922, 1923

Decatur Public Library has 1871-72, 1881, 1884, 1889, 1896, 1899, 1901, 1903, 1904, 1905, 1906, 1909, 1910, 1911, 1912, 1915, 1919, 1920

Don't assume that local repositories have a full collection for their own city! Indeed, since these books were ephemeral, more years may yet be out there.