Please, go do Halfus/Conclave/Magmaw and any other fight that involves adds on heroic on both 10 and 25m and tell me which is the hardest. Halfus, you can stack 4ish disc priests and 4 tanks on 25man, making the encounter piss easy. Conclave is just ridicilously overtuned in 10man atm, and i'm pretty sure magmaw isnt even doable in 10man. Every 25m kill they got a frost dk kiting the paracite and a kiting the big adds team, whilst everybody else is smacking the boss. Tell me how thats doable in 10man?
Organizing a 25m raid is harder, and the margin of error is bigger, but also less important. A dps can die in a 25m kill, your screwed if it happens in 10m.
Like I've said before, every single top guild in the world would be running multiple 10man groups for world firsts if they were so easy compared to 25m as people claim. But they aren't.