From what I gather most people run the Skunk2 alpha header. There was a thread I saw on here last week of people discussing the DC header. DC had made one of the first headers for the 06-11 Si and if I remember correctly some of the posts said DC has revised the header several times making it better.
If you don't want to research much then the Skunk2 alpha will certainly provide you great results. I'd be interested to see how the newest DC header would fair out against the Skunk2.