According to Play$tation store description...

Servo-skull: Take a Servo-skull with you on your missions. These are made from the skulls of the Imperium's most loyal servants as a reward for their devotion, and will follow you until your last breath..

Cherub pet: A Cherub is a bio-sculpted, flying  homonculus created to assist Inquisitors. They have been intentionally sculpted by the Adeptus Mechanicus to symolise the angels from the ancient Terran myths and they represent purity in the eyes of the Emporer: the perfect companion for thhose who are doing the Emporer's holy work in the Caligari Sector.  

The description basically sound like a cosmetic item NOT worth a real money transaction in the case of the multiple skulls on PC. 

I may be knew to Neocore's definition of "pets" in games but I'm use to pets that at the very least gather gold for you as in Diablo III which are free in game drops or rewards. I have seen no difference in running missions with and without the Cherrub equipped. 

If I'm missing something please let me know. Also I play on Xbone and dont have any reference tabs on console