In case you were wondering, here is an answer:
Some features could be merged, but there is an important difference in that org-ref uses bibtex as the backend database, and reftex for searching, and org-bibtex uses org-mode headings as the backend database, and tag/property searches (I think). It is like the difference between org-contacts and bbdb. They both serve similar needs, but with different data sources, and different ways to think about it.
Both approaches are quite nice. Sometimes it seems easier to be able to share the original, not exported, database with folks even though technically it makes no difference what is the system or origin for that data!