Triggered meaning they work the way they did in ulduar, where its the same instance, but changes mechanics to make the fight harder
Toggled means you just click your portrait and switch to Heroic raid
personally, i think triggered is just generally better
blizzard cited communication being easier with toggled hard modes, where it can be sometimes hard to get all raiders doing hardmode mechanics
personally, I think that makes little sense, If your the kind of guild that's doing hard-modes, you wouldn't have the kind of raider who wouldn't understand changing mechanics. (and if hardmodes are tuned so you can get away with that, then blizzard is doing it wrong)
i really think the the Triggered in raid hardmodes are better, but I'm always open to hearing different
think of it like a petition, if blizzard realizes people prefered to trigger them, we'll get em back =D