As Gary said the 1926 set is many times harder. IMO the 1926 standard set was cut short because of the popularity of the 1926-29 PC set, this short print makes it the toughest exhibit set to complete (the 1933 set is close just has a lot less cards needed).
Also the 26 blue is very different than the color used on the 1926-29 PC exhibits, examples below-
Examples-
1925
1926
1926-29 Blue
