Check the link in the first post. It's a blog post by Conor Cunningham, former member of the SQL Server Query processing team and former development lead on the Query Optimizer. He explains why there's a (very, ver, very small) difference
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability