This article by Kalev Leetaru (First Monday, Volume 13 Number 10 - 6 October 2008) compares these two flagship mass digitization efforts. From the abstract:
While the academic community has lauded OCA’s “open” model and condemned the proprietary Google, all is not always as it seems. Upon delving deeper into the underpinnings of both projects, we find Google achieves greater transparency in many regards, while OCA’s operational reality is more proprietary than often thought. Further, significant concerns are raised about the long–term sustainability of the OCA rights model, its metadata management, and its transparency that must be addressed as OCA moves forward.