Honestly difficult to say. The bulk of my raiding experience in original Wrath was 10-man content. I definitely agree with you that there’s a lot more individual pressure in 10-man. Not only because each player’s damage/healing contribution makes up a bigger fraction of the whole, but also because of things like more limited resources. Maybe you only have 1 battle rez, or sometimes even none at all. The fact that classes had started to be homogenized in Wrath alleviates this to some degree, but having a healthy diversity of classes and utilities just seems easier in 25-man. That feeling of extra pressure adds a lot to 10-man for me.
I will say that the scale of 25-man definitely feels a lot more epic. In a way, it almost makes it more satisfying seeing 25 players moving in unison rather than 10.
Wrath Classic so far has been my only experience with raiding “seriously” in 25-man, and so far I like it a lot. Though at this point, I don’t think I could choose which is my favorite. They both have their charm as Smáshing says.
To me, it simply seems like it gives better rewards because it’s more difficult from a logistics point of view. Especially where complicated mechanics are concerned, a lot more can go wrong in a 25-man raid. If we look at success as a gradient, or a scale of 1 - 100, for example, there’s a lot more granularity to a raid with more people. In 10-man, if you lose both tanks or both healers, or just the right number of DPS, calling it a wipe isn’t nearly as difficult of a decision to make. It seems to me like there’s a lot more potential for a 25-man raid to barely hang on and pull out a clutch victory, which means that there may be more potential for a more difficult fight.
Also, there’s no way to prove this really, but maybe 2009 Blizzard just looked at 25-man as being the “real” raiding content and therefore gave it the “real” gear. 