Are most NA guilds Horde?

I want to raid and am trying to figure out what the best side is for NA raid times.

Thanks in advance!

According to the posts on a recent thread, yes.

http://forum.warmane.com/showthread.php?t=391178&page=2

And if you look at retail, Horde is generally more popular in NA servers than EU. (but Alliance players still dominate in NA servers)