As the Assassin gains levels and becomes more experienced as a player and has better gear, Warrior might be a better choice in some situations, allowing you to tweak what skills the shadow uses. Using which shadow depends on preference and situation. Also, against enemies that are capable of destroying the shadow minion in a few hits, the fairly long casting delay on this skill will prevent the Assassin from refreshing her tank in time. The Master's fondness of spamming Mind Blast can disrupt a martial artist assassin's rhythm, as it may convert the monster that the assassin is charging up on, causing her to end up running aimlessly in place while trying to click the now untargetable monster, or maybe even straight into the heart of a horde of monsters. For trap-oriented assassins, the Master's traps share the same 5 trap limit, so she may end up laying a relatively harmless Charged Bolt Sentry that overrides a deadly Death Sentry. On the other hand, the Master's ability to use all skills can be a hindrance. The Master can also use skills that the assassin does not possess - for example, it can throw death sentries for a martial artist, or use Phoenix Strike for area of effect freeze for trappers. Overall the shadow master is stronger and tougher, with a random assortment of items, which can gain magical bonus as the skill level grows, including ones that boost skill levels, or resistance boosts that are capable of giving the Master immunity to certain elements (her resistance does not suffer from Nightmare and Hell's penalty, nor are they capped at 75%). Though this can be useful, using Master instead of Warrior does have pros and cons.
The upgrade to Shadow Warrior this skill allows the shadow to use skills the Assassin herself might not even know. The Shadow Master has access to all of the Assassin skills, also skills you don't have yet. Hence, it's possible to get the Shadow Master immune against elemental attacks.This discipline allows an Assassin to project an even more powerful shadow avatar. Resistances of the Shadow Master do not suffer from the penalties in Nightmare and Hell difficulty.You can either have a Shadow Master or a Shadow Warrior.The skills a Shadow Master casts have the skill-level equal to half her skill-level +1. Shadow Masters have a base strength and dexterity equal to their level * 10.Most players choose the Shadow Master for play past level 30 although the Shadow Warrior offers more control over which skills and spells she casts. The Shadow Master has Resistances when compared to the Shadow Warrior.Summon a Powerful Shadow of yourself to fight by your side.The Shadow Master has access to all of the Assassin skills. This discipline allows an Assassin to project an even more powerful shadow avatar. Some Skills have synergies, which provide benefits to other skills on the same Skill Tree. Skills usually define a character's playstyle and are divided by Class and Skill Tree. Shadow Master is part of the Shadow Disciplines Skill Tree and requires level 30 to be unlocked. Shadow Master is an Assassin Skill in Diablo 2.