Both scare the crud out of me, but I might have to say Colossi in this instance. Maybe it's just because their nastiness is the one I experience the most recently.
Even if you have Corruptors for cost (two for every one Colossus), they can't take down Colossi faster than the Colossi can rip apart your ground army, leaving a good deal of the Protoss ground support (Stalkers/Sentries) alive to kill your Corruptors.
Neural Parasite works better on Colossi than HT (Colossi are easier to click on), but it only works up to a point. Since Neural Parasite has the same range as the Colossi's attack, and since Colossi tend to rain death from behind the Protoss frontline, that means that your Infestors need to be on the Zerg front line in order to actually Neural Parasite a Colossi, and that means that it's going to be dead before it gets the chance to do so, or killed shortly after.
Yeah, you could try and get a flank in, but it's really fricking hard to do on many maps, because a lot of them are chokefests. And the Protoss army can move pretty quickly. If you're not completely prepared, you'll have one portion of your army engaging the enemy prematurely, before the other half can help.
I think that Incineration Zone is especially nasty. In contrast, it's amazing how much supply Protoss can cram into one compact ball - especially since Colossi can walk over the rest of your army. They effectively make it nigh impossible to snipe High Templar, if he has any hidden in his ball.
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