This truly changes the game when it comes to document collaboration. Whereas most collaboration tool providers focused their engineering resources on how to resolve conflicts between conflicting edits, Google decided to figure out a way to avoid conflicting edits altogether. Without Google's background and experience in running a large, multi-tenant, cloud-based offering (something Microsoft hasn't even done yet), intractable problems like these would be more difficult to solve.
That said, just looking at what Microsoft has been showing as a part of its technical preview of Office Web Apps (the one associated with Skydrive), it is very clear that Microsoft's only enemy in terms of delivering something that will meets many customers' needs (especially existing ones), is time.
Advantage: (for now) Google
Scenario: What else is in the box?
One reason it's impossible to do a true apples-to-apples comparison of Google to Microsoft is that their offerings differ so much in terms of what else is thrown into the box. If the closest comparison to Google Apps will be Microsoft's BPOS-S 2010, both have additional features and functions that I haven't begun to describe.
Some of these -- for example the ability to create intranet and Internet Web sites -- go head-to-head with each other. Both have an enterprise-class email server, too (one that Outlook works well with). Sharepoint has social networking features that Google Apps completely lacks. The Gmail service that comes inside of Google Apps includes a world-class anti-spam engine (courtesy of Google's acquisition of Postini). With BPOS-S, users get integrated unified communications by way of Office Communication Server. I could go on. I won't.
The question for you is whether or not you've vetted the list and checked off those things that are included that are must-haves versus nice-to-haves. Then, for the must-haves that one or both of the solutions lack, how much more will it cost you to get them.
Advantage: It's your call.