Here is a nice write up on Sharepoint vs Drupal. This is from microsoft blog so you might want to take this with a pinch of salt. But still it is the only blog in internet where someone is presenting the advantages of Sharepoint. Whichever blogs you might get by googling Sharepoint vs Drupal, they all speak about Drupal being best.
Updated: I recently came across this blog on Sharepoint 2010 vs WordPress. This is definitely nice read and can help decide which product to choose.