Which alternate upgrade for Inferno do you like the most?

Why is that?
Darkness vs infs is just the thing if you're playing as humans or necros.

and I didn't even mention darkness at all, I was specifically talking about same-//diffusion. The main use of it isn't blocking retaliation, but rather messing with shooters. I think it's better to spend a cast on blindness, or mirror, or... or..., than to block one stack of temptresses, who are by no means as strong in shooting as Arabs or elves to pose a serious threat.
...and this also applies to subjugation) specifically in the sense that it's better to use it on melee infs, so even here immunity doesn't solve anything.

And imps can steal mana even without pit lords—it just vanishes into nowhere. So if someone cares (and sometimes they do) about their +1 speed compared to devils, you don't even have to look at what grade of pits are in the army.

completely fair, it's just that in a confrontation with magical Factions I would still bet on imps rather than +1 speed^__^.

p.s. an attack of six cells is powerful, but that's a Dragon Utopia. What a shame :-(