CMSWire has a nice little article on Google Sites and MOSS. The general gist is that, while not as capable as MOSS, Google Sites may be just good enough for basic websites and collaboration. The absence of some advanced features may be compensated for by the simplicity of the platform. Sometimes just good enough is exactly what you want.
Over on the Sharepoint side, I just read Michael Sampson's excellent report The 7 Pillars of IT-Enabled Team Productivity: The Microsoft Shareport 2007 Analysis. The report uses Michael's 7 Pillars framework to evaluate Sharepoint. Not to be a spoiler but MOSS doesn't do so well.
I am not convinced that you want one platform to try do do everything but Michael assumes that this is Microsoft's goal (it certainly isn't building a robust WCM system). So, it seems that Sharepoint is caught in the middle. It is not the silver bullet, one-stop-shop for enterprise collaboration, and it is not the simple, cuddly tool that everyone loves to use. That is not a very fun place to be.