The only thing that matters is the mana drain itself. If they can drain mana unpunished for a couple of turns, then Imps are the way to go. Otherwise, it's Cherubs.
It says "a couple of turns." I'm saying that a couple of turns is not enough.
Let's say we have 200 Imps and 200 Cherubs. On the first turn, the Imps steal 50 mana from the Necromancer and open a gate. On the first turn, the Cherubs steal 25 mana and don't summon anything. On the second turn, the Imps become an independent combat unit (by the way, they are not as vulnerable to general weakening as the Cherubs). 80 Imps are summoned (with skillful gate opening). The Cherubs steal another 25 mana (if they aren't accidentally hit by Archliches, the Ring of Cold, or some other general plague, if they even get a chance, because Necromancers like to lower morale). On the third turn, the Imps are still fighting, if they survived, of course. The summoned Imps steal 20 mana (if they haven't been wiped out before the turn) and run into battle. The Cherubs steal the same 25 mana, and they simply don't have time to summon anything.
In total, by the third turn, the Imps steal 70 mana, while also being able to fight. By the third turn, the Cherubs will steal 75 mana if their number hasn't decreased. That is, for the sake of argument, on the third turn, you exchange 280 fighting Imps for 5 extra units of mana (and if the Necromancer feels the need to interrupt the mana drain, he will simply blast the Cherubs with bone arrows/Archliches/magic). In my opinion, it's not worth taking Cherubs just to deprive the opponent of mana – they need to be protected, and it's not worth losing a combat unit for dubious gain. By the way, it's unclear how you can determine in advance whether the Cherubs will be able to stand for several turns or not – it all depends on the Necromancer's desire. I can't imagine a battle in which the Necromancer allows his mana to be stolen for more than three turns if it's critical for him.
If there are a lot of Imps by chance (for example, creature dwellings, no losses of Imps against neutrals, joined troops), then due to their speed and with some randomly obtained divine power, the Cherubs can be an excellent unit. Also, if the armies are not too large, and the Cave Lords can still change things with their magic, then the Cherubs will be useful. In all other cases, Imps are more reliable, as they don't miss turns.
P.S. I don't quite understand how Cherubs will prevent the Necromancer from using Spiritual Link? It gives a lot of mana with large armies, and the hero is only pushed back by half a turn. And if the armies are small, then the Cherubs won't steal much mana either.
Besides, you're only looking at Dark spells cast by the Necromancer. Has Chaos been officially banned? Necromancers quite like the Ring of Cold with cold perks.
