When using this command:

/give @p mob_spawner 1 0 {BlockEntityTag:{SpawnPotentials:[{Entity:{id:"Cow"},Weight:1}]},SpawnData:{id:"Cow"}}

They will not go together, if you put SpawnPotentials in front, it will use SpawnPotentials on the next turn instead of the Cow spawning then switching to more cows spawning. When SpawnData is placed in front of SpawnPotentials, it will ignore SpawnPotentials and continue spawning Pigs as the default choice.