Organic Groups vs Group - on D7 vs D8? I've done some research on this question, posted it here without any feedback, and have left messages for OG owner (amitaibu) without any response, so hopefully I can solicit useful feedback here. 

I'm building a story telling platform that combines passions for genealogy,
family photography, and social media, and brings them to a whole new and
integrated level of telling stories about your ancestors, and celebrating the
life of an individual who has just passed away. I'm building this on Drupal
with Organic Groups as the main organizing construct. Every Album - whether
it's a family tree, a single individual, or even a photo album - is
instantiated as an OG. I'm writing about it and will launch it at
www.greatalbum.net.

Right now, I'm building the prototype on D7+OG because it's stable and it's essential that any piece of content (family/individual/event/place/story/media/etc.) be allowed to exist in more than Album. The goal
is to launch the prototype in April, and then launch the production v1 
site in June or July. I'd love the public launch to be on D8+OG, but I'm not
sure how realistic that is, since OG v8.x is currently 1.0-alpha2.

So, my questions are:


Am I correct to understand that Group module does not allow content to exist in more than one group at a time, and hence based on my requirement OG is the better choice?
Should I assume that OG 8.x-1.0-alpha2 is not stable enough to use for a production site, and that I should stick with OG 7.x-2.10 on Drupal7 for now, and wait on the benefits of Drupal8 until OG matures at least to RC1? 


Any guidance?
... Am I correct to understand that Group module does not allow content to exist in more than one group at a time ...


Yes that is correct, but you seem to not take into consideration that the Group module has the notion of Group Types, and when you create "a" Group, you have to specify which its Group type is (compare to creating nodes, which you can only do if you first specify the content type for that node).

Moreover, a user can:


have multiple levels of access (none, read, update) to some group (outsider, member, etc).
be member of multiple Groups.



  ...  and hence based on my requirement OG is the better choice?


That's a matter of preference (opinion based), e.g. if you prefer to go for D8, at this time, IMO, the Group module is way more stable ...


  
  Should I assume that OG 8.x-1.0-alpha2 is not stable enough to use for a production site, and that I should stick with OG 7.x-2.10 on Drupal7 for now, and wait on the benefits of Drupal8 until OG matures at least to RC1? 
  


If you do decide to go for OG, then for the sake of stability, at this time, D7 seems to be the way to go.

Read more: What are the features of the Group module versus Organic Group module?