Postgres is so amazing I still find it hard to believe it's free. I abused it many times (as a graph database; as a real-time financial data analytics engine with thousands of new ticks coming in each second; as a document storage with several TBs of data per node), and amazingly, it just worked. Magic.
If in doubt, choose Postgres.
Though I stored document relationships and social graphs, not semantic graphs, as they describe in this paper. I have little experience with semantic graphs — perhaps, they have different requirements.
Joins are good. :)
While I was listening this presentation I've thought that it's quite weird functionality and intersects a lot with transaction visibility rules that MVCC already has. So it is interesting what is your use case?
PostgreSQL is only free if your time has no value..
(Paraphrased. In jest..)
I worked with a certain other database from a company starting with "O" and ending with "racle", and support/documentation quality was abysmal. Our support contract was quite basic, but still, it expected to be better than free alternatives.
It was very NOT.
But in comparison to an enterprise solution like DB2 or Oracle? Not even close...
If you are trying to pull nosql into the argument, that is not even apples to oranges, plus with nosql, you get to retool your entire architecture.