Sacred Weapon vs Vow of Enmity

 Of the four Paladin subclasses in the new Player's Handbook, the two that appeal to me the most are two of the most "basic" ones - Oath of Devotion and Oath of Vengeance. While I was tempted by the Oath of the Watchers in Tasha's, I've generally not been all that impressed by the subclasses that came out in the various supplements (Oath of Glory might be my least favorite, and I was kind of disappointed to see it in the PHB instead of something off-beat like Oath of Conquest).

Paladins get a lot of amazing features as base class features, so I think, honestly, that any Paladin can be quite good. But both of these subclasses grant you a Channel Divinity option that aids in your ability to hit targets.

In 2014, these two features had very different action-economy costs. Vengeance's Vow of Enmity cost a bonus action to activate, which allowed you to pretty easily use it on your first turn and then unload on your target. And Smites didn't take any action, so you could come right out the gate with a smite on two attacks made at advantage.

Sacred Weapon, however, was a bit frustrating - it took your full action to activate, meaning that even though you were now more likely to hit, you'd spent a turn just gearing up. If you could get this off before a fight, it was quite good, but with only a single minute duration, you really needed to be quick on the draw and have a generous DM.

Now, however, the action economy of both features works the same - neither takes any action, and can just be activated when you take the Attack action (which is when you'd want it anyway). Let's get specific on how they work:

Sacred Weapon allows you, when you take the Attack action, to expend a use of Channel Divinity to supercharge the weapon you're holding with positive energy. While charged this way, you can add your Charisma modifier to attack rolls made with the weapon, and if you want, the weapon can deal Radiant damage instead of its normal damage. The weapon also glows, shedding bright light for 20 feet and dim light another 20. And you can end this effect early whenever you want (it also ends if you aren't holding the weapon anymore).

Vow of Enmity allows you, when you take the Attack action, to expend a use of Channel Divinity to utter a vow of enmity against a creature you can see within 30 feet, which lasts for 1 minute. While under the effects of the vow, you get advantage on attacks against that creature. If the creature drops to 0 HP while under the vow, you can transfer the vow to another target within 30 feet.

So, before we get into the math, let's make a few observations: The first is that Sacred Weapon lasts ten times as long as Vow of Enmity, and while ten minutes isn't a lot of time, it's possible that you could get use of it through multiple combat encounters - though probably only occasionally unless you're in true old-school dungeon-crawl mode. Another consideration that favors Sacred Weapon is that the effect is on your weapon, and so it will remain there for the full duration regardless of how many enemies are nearby. One point away from it is that if there is an enemy that can disarm you, the effect can end early.

But let's talk about the damage potential of each.

General wisdom for 5.5e is that it's far easier to get advantage on attacks than it used to be, which might devalue Vow of Enmity a little.

However, if we look at this in a relative vacuum, we should figure out the extent to which this will increase our damage.

Generally, my assumption for most Strength-based characters is that if they want to deal the most damage, they're going to do a Great Weapon Master build, favoring a Greatsword, and possibly switching to a Glaive if they pair it with Polearm Master, because these weapons have the Graze property. Admittedly, we're looking at features that make us miss less often, which devalues Graze, but I still think it's nigh-unbeatable for damage potential among weapon masteries (though Vex and Nick are also strong contenders).

I'm going to run this simulation at level 8. Granted, Paladins get a serious damage boost at level 11, but 8 is where my current longest-played character is at, and also feels like a good level for "meat and potatoes" D&D.

We'll assume Point Buy and a background that has allowed us to start with a 17 in Strength and 16 in Charisma. At level 4, we've taken Great Weapon Master, and at 8, I think we probably have taken Mage Slayer - meaning we're at 19 Strength, and thus a +4 to Strength and +3 to Charisma.

Now, what AC are we dealing with? While Channel Divinity is not that hard to get back, I'm going to assume a slightly tougher foe. And surely there are few better foes for a Paladin than a Death Knight. Now, a Death Knight proper is a bit high-CR for a level 8 party - they can work as a boss at the end of tier 2 for 10th level characters, but this might be a bit much. Luckily, we've got a lower-powered Death Knight in the Aspirant, which could be a scary but ultimately manageable powerful foe. And both have an AC of 20.

Generally, I've done a lot of my calculations here of late assuming only mundane weapons. I do think that by level 8 I'd hope your DM would have given you a +1 weapon at least. I'm not entirely sure how this would change the math. Still, it's easier to just assume mundane and move on from there.

Now, here's another complication: a Paladin, especially fighting an undead or fiend target, is probably going to burn their bonus action on a Divine Smite if they land a critical hit. Thus, we might actually ignore the Hew element of Great Weapon Master (they still might use the Hew attack when downing a lesser foe).

Initially, we'll ignore Divine Smite. This is likely to favor Vow of Enmity, as it increases our chance to crit, so only if Sacred Weapon does better will we take it into consideration.

Baseline, our Paladin has a +7 to hit. Sacred Weapon will boost this to +10. Thus, the Vengeance Paladin (before accounting for advantage) has a 40% chance to hit. The Devotion Paladin has a 55% chance to hit.

Damage on a hit is 2d6+7 (getting an extra 3 from Great Weapon Master), for an average of 14

Crit bonus damage is 2d6, or 7.

Thanks to Graze, we also do 4 damage on a miss. What this actually means is that we can subtract the 4 from the hit damage and just add a raw 4 to the average we get in the end. Thus, we can ignore misses when calculating the chances and just treat hits like 2d6+3, or 10.

We'll start with Devotion.

10x55%, or 5.5, plus 7x5%, or .35, gives us 5.85. We then add in that 4 damage we get 100% of the time, giving us 9.85. Double that and we get 19.7

Now Vengeance.

Thanks to advantage, our hit and crit chances are a little different. With a 40% chance to hit on any given die, that gives us a 64% chance to hit (and already, our question might be answered, with caveats). We also have a 9.75% to crit.

Thus, we take that same 10x64%, giving us 6.4, plus 7x9.75%, or .6825, for a total of 7.0825. We then add in that 4 from Strength, giving us 11.0825, and double it to 22.165 for our two attacks.

Rather than continue, it becomes pretty clear that Vengeance has the edge, and will further expand that edge thanks to more opportunities to crit-smite or get off Hew attacks.

And yet...

This is all under the assumption that the Paladin cannot get advantage in some other way. If an ally gets off Faerie Fire, or the foe can be repeatedly knocked prone, Vow of Enmity loses its value, while Sacred Weapon will continue to increase your hit chance in all scenarios.