Information about Creature Types can be found here. More specifically, here are direct links for Aberrations and Monstrous Humanoids.
Statistical differences include
- Hit die is d8 for Aberration vs d10 for M.H.
- BAB is 3/4 for Aberrations vs Full for M.H
- Saves: Aberrations have “Good” Will Saves while M.H. have “Good” Reflex and Will Saves (both have average for the unlisted Saves)
- Different Skills counted as “Class Skills”
Just as important are the descriptions of the creatures.
Aberrations are described
An aberration has a bizarre anatomy, strange abilities, an alien mindset, or any combination of the three. An aberration has the following features.
while Monstrous Humanoid are described
Monstrous humanoids are similar to humanoids, but with monstrous or animalistic features. They often have magical abilities as well. A monstrous humanoid has the following features.
By this description, Centaur might have been aberrations, but the fact that they came about in a natural way and their mindset being similar to that of the humanoid races suggests that M.H. is the better classification for them. Driders’ unnatural origin makes the difference.