The nine pellet 00 loads are lower recoil than the eight pellet 000 loads. While one pellet might not seem like a big difference, you have to consider the weight of the pellets. A single 00 Buck pellet weighs approximately 53.8gr while a single 000 Buck pellet weighs approximately 70gr, so the total weight of a nine pellet 00 load would be about 484.2gr while the total weight of an eight pellet 000 load would be about 560gr. If we look at velocities, we find that Federal's Power-Shok nine pellet 00 load and eight pellet 000 load have the same advertised velocity of 1325fps. Because the heavier weight of the 000 loading will give use higher energy and momentum, it will also have more recoil assuming both are fired in the same shotgun.
Bear in mind that the whole point of nine pellet 00 loadings is reduced recoil. If recoil is not a concern, Federal also makes a Power-Shok 12 pellet 00 loading with an advertised velocity of 1290fps, but that loading has more energy and more recoil than either the nine pellet 00 or the eight pellet 000 loads do because of it even greater combined pellet weight of 645.6gr.