I also think of this in terms of viable development migration paths:
For example, 9.3's upcoming: hstore_to_json(hstore) and hstore_to_json_loose(hstore) open up possibilities. If an hstore datatype is chosen now, you can feel fairly confident that it can be fairly straightforwardly migrated to a native json datatype in 9.3 or later versions, when you feel the json featureset and stability may have become mature enough for your particular project's needs.
For example, 9.3's upcoming: hstore_to_json(hstore) and hstore_to_json_loose(hstore) open up possibilities. If an hstore datatype is chosen now, you can feel fairly confident that it can be fairly straightforwardly migrated to a native json datatype in 9.3 or later versions, when you feel the json featureset and stability may have become mature enough for your particular project's needs.