Which is likely to provide more chances to opportunity attack, Polearm Master or Sentinel (with War Caster)?

Question

I'm wielding a staff focus, which doubles as a quarterstaff; so, it seems that Polearm Master + War Caster allows a PC to cast spells as reactions to enemies that enter its reach.

Similarly, Sentinel + War Caster allows a PC to cast spells as reactions, even when the enemy takes the Disengage action.

Three questions:

1. Can I combine Polearm Master/Sentinel with War Caster to cast spells as opportunity attacks when an enemy enters my reach (PM) or when they Disengage from reach (Sentinel)?
2. If I want to maximise how often I can use my reaction to opportunity attack with spells, which is the better pick, Polearm Master or Sentinel? On the face of it, it seems like the former, but I've only started playing, so, not much experience yet.
3. If Polearm Master is superior (as described above), is there a way to compare the value of Sentinel's "stopping effect" (on a hit, you set the enemy's speed to 0) vs. the added damage? The added element of battlefield control can be quite valuable (though hard to quantify).

Answer 1

1. Can I combine Polearm Master/Sentinel with War Caster to cast spells as OAs when an enemy >enters my reach (PM) or when they Disengage from reach (Sentinel)?

(note: technically speaking, all of this will be up to your DM)

War Caster with Sentinel, creatures moving away even with Disengage, yes; creatures within 5' attacking a different target no: Sentinel specifies melee weapon attack.

War Caster with Polearm Master, RAI: no, see this link: Mearlsian Q&As

Do Polearm Master and War Caster combine to allow a magic user to make a spell >opportunity attack when they enter reach? No - polearm master applies only if you use the >weapons it lists to make the attack

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2. If I want to maximise how often I can use my reaction to OA with spells, which is the >better pick, Polearm Master or Sentinel? On the face of it, it seems like the former, but >I've only started playing, so, not much experience yet.

If played/ruled RAI, then clearly Sentinel, but if Polearm Master/War Caster spell casting is allowed, it would still very much depend on the DM and campaign/adventures; for example many creatures with the ability to Disengage as a bonus action would heavily tip this in favor of Sentinel.

3. If Polearm Master is superior (as described above), is there a way to compare the >value of Sentinel's "stopping effect" (on a hit, you set the enemy's speed to 0) vs. the >added damage? The added element of battlefield control can be quite valuable (though hard >to quantify).

Again, this would be far too dependent on the DM/adventure to realistically quantify with any reasonable confidence.

Answer 2

The rules as written support the use of sentinel/polearm master + war caster to work as you intend. Balance wise this also seems fine to me as it costs you two feats in order to achieve this. Since you are playing a sorceror that means you have access to 5 feats or ability increases, assuming 2 will be used to max charisma and 1 for elemental adept that leaves you with this combination as your remaining character improvements.

Of note for the Sentinel feat, the OA granted when a creature in melee attacks someone other than you specifically states your reaction can be used to make a MELEE attack, meaning no bonus spell in this situation. Sentinel would be useful only for when creatures attempt to flee, making it less desireable because the range of spells allows you to hit once or twice during their escape anyway.

You would get more bang for your buck using the combo of polearm master and war caster since it activates anytime a creature comes within 10 feet of you.

As for War Caster, this permits you the somatic components of spells while wielding a weapon only. Material is covered by your focus quarterstaff. However, any spell requiring material components that are consumed do not use your focus. This means you would have to stow either your weapon or shield in order to handle them, if you choose to stow weapon then polearm master would no longer apply. This balances out your combination choice in that some powerful spells won't be able to be cast in this fashion.

Answer 3

If you want to increase frequency then Polearm Master as it grants an opportunity attach when an opponent enters your reach. A more frequent occurrence than a opponent using a disengage action to leave your reach.

The other benefit of Sentinel is allowing you take a reaction to make an attack action on an opponent within 5 feet of you while attacking an ally. This is not specified as an Opportunity attack. So you can't use War Caster with this ability. A ruling from Mike Mearls may change this.

Polearm Master or Sentinel with War Caster.

The war caster feat states that the caster may use his opportunity attack. It does not limit this to specific instances of opportunity attacks. Since both Polearm Master and Sentinel expands the opportunities for taking a reaction to take an opportunity attack they also expand the opportunities for casting a spell with a reaction using war caster.

From Page 170 of the D&D 5e PHB

War Caster

When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack.

And

Sentinel

Creatures within 5 feet o f you provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

From Page 169 of the D&D 5e PHB

Polearm Master

While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.