They're cousins from the same code base originally.
My guess is that PostgreSQL is cleaner, but missing some of the features ingres might have gotten from being commercially developed.
However, in terms of quality of code, I doubt it has any advantage over PostgreSQL.
Plus, PostgreSQL probably has a larger installed base, as it's been free for quite some time.
PostgreSQL has a very large, very capable community behind it. Ingres has a long way to go before it can equal postgresql in that arena.
