-Horde is a bit more successful in PvE. If you're a hardcore, serious raider who's always on top of the progression, the current best guild is Alliance. But, for pugs/casual raiding, Horde is better.
Yeah, top guild may be Alliance, but all the other top guilds behind it are basically Horde.
Yeah, I know. That's what I just said.
The way you worded it made it sound like "Alliance is better for hardcore raiding, Horde for casuals".
Horde-side probably makes up the majority of the hardcore raid guilds.