As most readers know, I'm a 25-man raider. It's not that I dislike 10s, it's more that I just don't care about them. A lot of other people do care, though, enough that's it worthwhile to have 10-man raids exist.
But to me, 10-man raids are UBRS1. I did UBRS a lot, before I started raiding. It was fun and all, but it was just UBRS.
25s are Molten Core and Blackwing Lair. The grail, the goal. When I think of 25s, it seems to tap into that line of instances.
I'm not really going anywhere with this train of thought. But really, I don't think there's any real, logical reason I prefer 25s to 10s.
I do think that for the Royalty guilds, the Paragons and Premonitions, 25s will provide a greater challenge than 10s. But for someone at my level, I think a tuned 10-man can be just as challenging as a 25-man.
I do like working within a sub-team, having a healer channel, etc. Yet it's also nice being part of the entire team.
But still, at the end of the day, 10s are UBRS and 25s are Molten Core or Blackwing Lair. The patterns laid down in our youth are hard to break.
1. Upper Blackrock Spire. But really, it's UBRS.