It just depends on your playstyle, mid shadow is very versatile right now. If you want to go aggressive consider running a version with phantom howl and cerb, if you want to play into the lategame play a list with mordecai and immortal thane, if you want to swarm the field and create a really sticky board play a list with catacombs and death's breath. It's a matter of preference, really.
It has midrange in the name for a reason. Catacombs are a stupid idea , same as death's breath since you fuck your shadow pool with that a bit.
I do not like people like that to be honest. You are taking the best deck arm and even then want others to fix the details. However, to put it simply and maybe help you out in the future. Do not look at the differences, look at what is in every single one, then you have a core. Now check the cards that are different and try to figure out what they bring into the gameplay like in this case the main players are Thane and Cerberus. Thane provides a safety plan sort of plus a slowly bleeding out for your enemy. Cerberus provides you with even more burst and aggression earlier. So should you run cerb or thane ? Or maybe both or neither ? The thing is there are too many factors to determine which version is the best :
Meta is slightly different on different tiers so a deck which is crushing at let's say a may do much worse at master because the favourable match ups happen less often.
The personal preference of a player who runs what and which less obvious techs/ mechanics said player found appealing
Also notice that some decks have a huge discrepancy in win ratio on different tiers. One reason for that is the meta difference but also that some decks require higher skill to master so they work better on higher tiers.