With current D8 status, what's the decision tree for creating a new content entity type versus creating a Content Type for the "Node" content entity?
Decision tree for choosing between creating a new Node-entity-type Content Type versus a new content entity type:


Are you a programmer, or do you have easy access to a programmer?  


If no, then you are presently pretty-much limited to creating new Node-entity Content Types.  (There isn't a UI in core for creating new content entity types, and the Entity Construction Kit (ECK) contrib module currently only has an alpha release.)
If yes, then continue...

Do you know exactly which contrib modules you want to leverage for your data?  


If no, then the safe bet is probably to just create a Node-entity Content Type.
If yes, do those modules already support the custom entity type(s) you're considering, or are you willing to help enhance them so they will or recreate the desired functionality in your module?  


If no, then just creating a Node-entity-type Content Type may be best for you.  
If yes, then continue...


Will the actual content data that is enabled by your module merely help group or annotate other content data?  (So that it will seldom be viewed on its own.)


If yes, then consider whether your module is like the existing entity types for taxonomy terms and comments.  If it is, then a new entity type may be a safe bet.
If no, then continue...

Can you clearly articulate why a new Node-entity-type Content Type will not work for you?


If no, then your safe bet is to probably just create a new Node-entity-type Content Type.
If yes, then continue...

Lastly, are you sure that you can't just extend the Node-entity-type and that you want to recreate all of its useful functionality like CRUD operations?


If yes, then go ahead and create a new custom entity type.
If no, or if you're not sure, then you probably want to engage some experts to help you decide.



Notes:


This decision tree does not consider performance or security.  I'm not sure if or when a new custom content entity type would perform better and be more or less secure than a Node-entity-type.
Even if this tree is a good answer, it will probably not remain one over time due to updates in Drupal core and contrib module releases.  StackExchange doesn't seem to accommodate how the "best" answer today may not be the best answer tomorrow.)