Desolate Undead  

The reason that Desolate Undead is much better than it's live counter-part Reproach against undead is because it blows it any in damage per second over time, even if it does not beat it in single-shot damage. I will crunch some numbers for you below to show you what I mean. (For the purpose of this example I am using base casting times and damage modifier, but as you will see, increasing either of those will widen the gap between their dps)

First we will cast Reproach:
6.5 seconds go by
Boom - 1575 dmg
2 seconds go by as the spell gems refresh
Now we begin to cast Ancient Chaos Censure because we are assuming that you have done a lot of GoD to get to these spells and that would be the next best nuke you have at this point. You can not just cast Reproach again because you have a 7-second recast time.
6.5 more seconds go by
Boom - 1320 dmg
2 seconds go by as the spell gems refresh

Now all your spell gems are up and we have returned where we begun. Let's add up dmg and time and divide them for damage per second

1575 + 1320 = 2895 damage
6.5 + 2 + 6.5 + 2 = 17 seconds
2895 / 17 = ~170 damage per second
Also, just to throw it in there, we used 500 mana + 438 mana to cast those spells, or 938 total, which is 55 mana per second.

Now we look at the undead nuke Resolate Undead

We begin to cast Desolate Undead
5 seconds go by
Boom - 1560 damage
2 seconds go by as the spell gems refresh
And now we are back where we began. Let's do the same dps calculations:

1560 damage
5 + 2 = 7 seconds
1560 / 7 = ~223 damage per second

As a side note, look at the mana used (375) over 7 seconds is ~53.

Overall, the mana you are going to use per second is about the same. The difference is the dps achievable by using live nukes, which is ~173, compared to undead nukes, which is ~223. If you are going for maximum damage against an undead target, then the undead route is the way to go.