It seems like there are several active modules that all have similar functionality. Can anyone offer any information as to which branch appears to be the most promising / likely to survive & thrive? Any advice is welcome.
I've tried them all for an upcoming project. And the way i see it, only use Relation if you're dealing with complex relationships, not one-to-one references. If what you need is a basic reference, I'd go for references or entityreference.
I think that the last update concerning a widget for Relation, is that they're going to drop the idea, and consider Relation as an API only module. But who knows?
One note, if you want to use entityreference and field_collection together - it's buggy. Just fyi.
As of March 2012, the References module project page is now recommending Entity Reference as a replacement.
For basic node-type references, Entity Reference is certainly easier to set up than Relation. I think a good general rule would be
If you can do what you want with Entity Reference, use that. Otherwise, go with Relation.
Relation is planned as a replacement for References. Visit the Drupal.org/modules/references or /relation pages. Relation looks very good, but is not fully implemented yet. They have been working on widgets to create the relationships, but have also had some problems. Despite the limitations current widget I like the relation module capabilities. I have been working using both as I get more familiar with Drupal.
I recommend using Entity Reference & Views instead of Relation if possible. For those wondering about the differences, I wrote a blog post on how to install Relation as well as its advantages and disadvantages. I hope this helps some people.