They will both function as enemies as intended by the game - following pathnodes when directed, set active/inactive the same ways, etc. They will also use the same functions for calling out random dialogue if done so in the Model Editor's User Defined Variables.
The main difference between the two is that the Manpig allows for different standing positions. As I recall, the Manpig can be upright (biped) or can be on all fours (quadruped), swapping between the two when invoked by code:
void ChangeManPigPose(string&in asName, string&in asPoseType);
Requires 1.3
Changes the pose a specified ManPig.
asName - Internal name of the enemy
asPoseType- Name of the ManPig pose to use. Can be “Biped” or “Quadruped”
By adjusting values in the Model Editor, I believe you can change the speeds of the Manpig in these states (for example, slower in Biped, faster in Quadruped).
Therefore, you can create an enemy that has two different states; they do not necessarily need to be Bipedal or Quadrupedal, but that will be the code used (unless you create a custom function for them).