10 man, and 25 man are, according to blizzard, equally balanced in terms of difficulty (I.E bosses on 10 man should be roughly equal to bosses in 25 man in terms of time taken, mechanics, ect, per person)
10 man leads to greater responsibilty on the single player- A single dps/healer slacking in 10 man can be felt significantly greater then a single dps/healer slacking in 25 man. In addition, on cutting-edge fights, the death of a single player in 10 man can be the difference between a wipe and a kill- 25 man, the death of a single player can be unfelt, besides the fact that the boss was killed 5 seconds later (Which, when beating enrage timers, can make a ton of difference)
25 man leads to greater cooridination required- 15 more players requires greater coordination, more consumables per fight, ect. In 10 mans, the individuals skill as an individual matters- in 25 mans, the skill of a group matters more.
Because of that, I believe 10 mans and 25 mans to be of equal difficulty in the fight itself. Outside of the fight, 25 mans require significantly more coordination.
Also, its DREAM Paragon. They would be world first even if they raided 2 hours, 1 night, because they are a cut above everyone else.