1. ISP monitor all your internet traffic. Google can only have access to what google services you opt in to use.
2. At this time we don't know how the ISP's will sell your internet history whether it be individual or in aggregate, but google doesn't sell your information.
3. I trust google's security more than ISPs
This is, IMHO, one of the biggest unresolved ethical issues with data protection and privacy today. On the one hand, it would be a great burden on society to stop everyone from using any third party services for anything involving personal data about someone else. On the other hand, when those services are no longer neutral and will process any data they receive for other purposes without the informed consent of some of the data subjects, that's a dubious practice at best. Another example is a social network's mobile app uploading your phone's entire contacts list, and just like that they've probably got a unique personal ID and a great deal of networking information about everyone on that list.