the beasts differ in several areas that you need to look at..
Cats are singular target DPS masters, (at least they were) they can put out quite a bit of damage to a single target helping you take down stronger enemies faster.
The Gorilla is an AOE master and can allow you to target and hold several mobs at once while you god volley (AKA: Volley shot) them down to nothing.
The Gorilla helps with leveling quite a bit. For the stronger singular boss type monsters use your cat.
However this is all up to you since now most pets can be viable for most things. Choose what you like best.
Them: Shadow what? Me: Shadowkaizen..you will find out what it means soon enough..