25 man is significantly easier than 10man in many ways tbh....
For starters, you have all the people you need, tanking is less of an issue since you can afford to have that third tank.
Healing is WAY easier on 25 than 10 - tank dmg isn't that much higher, raid dmg isn't that much higher either - unless, ofc, half of your raid is filled with retards. That however, shouldn't happen in proper guilds...
Also DPS - seems to me - to be less of an issue - especially if you choose to go with 6 healers.
So alltogether; More buffs, easier healing, an affordable 3rd(maybe even 4th) tank - DPS seeming, to me, as less of an problem.... Well if you have 25 players that play well together and who think with their OWN brains instead of waiting for every single of the RL's commands... I'll probably go as far as to say that 10man by far beats 25 on difficulty.
Of course this is different from encounter to encounter, but alltogether this is my impression (Did you try Cho'gall on 25? - My lord he's easy!!)