Ive done this tier, both 10man(under protest) and 25man on heroic so in a good position to comment.
25man raiding is harder, pure and simple. Assuming you dont have 25 "world class" raiders. Which the absolute majority of guilds dont.
There is a single fight which is actually more difficult on 10 man, and thats hagara. Other than that single fight ie, logistically, encounter difficulty and socially, a 25man raid is harder to do. Its harder to fill a roster, its harder to get everyone to the right thing everytime (ok we all know the mechanics, but we are human and make mistakes). Its harder to organise, its harder for the officers. This is 7 years of experiance talking with both 10man, 20man, 25 and 40man raiding.
Most guilds are 10man because its easier, simple as that. Those that are 25 still, choose to be because they feel 10man isnt epic enough, a sentiment i share, despite the fact its more difficult without any extra rewards.