Humans are like electricity. Both of them will choose a path with least resistance.
10-mans are easier to organize.
10-mans usually have much better atmosphere compared to 25-mans. People are more relaxed in a smaller group of people => everyone knows each other better. *
10-man enrage timers aren't as tight as they are on 25-man difficulty. You can bring suboptimal raid group (or low DPS) and still succeed. **
25-mans drop slightly more loots for each person, but 25-man bosses still have a chance to drop 2 or 3 (??) times the same item. ***
10-mans do not need spare players. It is much easier to find 10 players with 100% attendance than finding 25 players is. Obviously there are exceptions.
It is much easier to deal with 10 players. Much less drama and problems with adapting to different tactics.
* Personal experience
** Enrage timers are same on both difficulties. Check
www.worldoflogs.com for statistics. Most 25-man kills are ~1 minute longer than 10-man kills are.
*** In my opinion 10-mans are in a better spot here. Obviously it is faster to gear up few guys on 25-man difficulty, but it still takes longer to gear up the whole raid.
And as a note; I am former 25-man raider (TBC -> T11) who converted into 10-mans when Firelands was released, and I think 10-mans are in a way better spot right now. I'd like to return to 25-player raiding, but running 25-man guild is too much trouble and not enough rewarding anymore.