One scrollbar or two for content + sidebar?

I'm working on a website design and trying to decide whether to have the sidebar scroll independently of the page content, or have just one scrollbar control both divs. What principle should I use to guide the decision? In which situations would you want the sidebar content to stay fixed while a user scrolls through the main content?

One Scrollbar

• User knows where they are on the page
• content in sidebar remains in same position relative to other content
• Holding the mouse anywhere on the page scrolls the bar
• Large majority of websites use just one scrollbar -- fits with user expectations
• can have fixed width sites without ugly scrollbar in middle of page

Two Scrollbars

• Can view information in sidebar while browsing main content, which is nice when the main content is >4000px in length, or you need information in the scrollbar and in the main content bar at the same time
• Sidebar requires either entire left hand side of page, or fixed header which reduces the importance of the main content, vertical height of both sidebar and main content
• Requires mouse placement inside sidebar to scroll its content
• Two examples of differing decisions here from the same company would be Google Code, which is a fixed-width, one scrollbar site and Google Reader, which has two scrollbars. I think with Google Reader they need a scrollbar because the content is dynamically generated.

I appreciate your help with this -- especially if you could point me to data leaning either way.