The conversation that is brought up from time to time is that, 25 is harder then 10 and vice versa. I have had the experience of being in a top-50 25 man guild and a top-50 10 man guild, this being said I will talk about my personal experience and talk about the pro’s and cons in my opinion.
I will start with why 25 man is hard. 25 man usually has another mechanic compared to 10 man, additionally 25 man has a history of having a harder time beating tight enrages compared to 10 man. Sometimes 10 and 25 man have the same requirements from their raiders damage wise and this is an advantage to 10 man in my opinion. In 10 man it is easier to manipulate the raid to increase damage as 25 man the only way to manipulate the damage is take out a healer or get some more gear.
10 man is more difficult when it comes to asking players to executing mechanics, as on 25 man 3 people can get resed and on 10 man only 1 can get reseed, so it is more vital to not make mistakes. Additionally mechanics are sometimes harder for 10 man players because there isn’t always a way around things like in 25 man. A prime example is heroic Warlord Blackhorn was one of the most difficult fights as each time someone needed to soak it had the be done by 2-3 players and the requirement was the same on 25 man this made it hard to execute the mechanics without the ship blowing although the beserker timer was virtually a non issue compared to on 25 man which was a DPS check.
Now that I have told my opinion I’ll let you decide which one is harder, as they both have there difficult sides.