i think the issue of groups vs. circles is not a big deal, really. The problem with groups, like you said, is that it requires work, and are difficult to leave. Also, groups came at a time when FB was getting a lot of negative press because of +Mark Zuckerberg
Google does have a limited window with which to catch peoples' attention on G+ though. I think they need to go stones to the wall with every feature, full and proper privacy, and be very in-your-face to gain a lasting amount of attention until it catches on.
The differences are few between the two, so anything google does is basically already being done by facebook. Google needs to do it better. Facebook has notes, Google has Google Blogs (soon.) Facebook has one on one video, Google has Hangouts AND Google Talk, Facebook has if anything finally been seen as showing up late to the party by just having now announced skype, while google has already had google talk and hangouts.
As long as google stays a step ahead, they are fine.