MySQL is better supported by shared hosts and easier to configure for novices. This is why MySQL was chosen from the start, not because it has any inherent advantage over Postgres.
Having used both, I can't see any salient and significant technical advantage that either holds over the other. In a point-by-point comparison they each win some points, but never by a margin that makes me favor one decisively over the other. Except for the fact that MySQL near-ubquitious on shared hosts, which is a pretty big advantage for deployment.
From the start we've planned to support database servers other than MySQL, but it has never been a priority. We'd like to support Postgres, for sure, since it is a solid alternative. In particular, I'd really like to support both MySQL & Postgres because it'll make for some interesting and informative benchmarking.
I'd also like to implement support for Microsoft SQL Server, to give Windows users a solid "native" engine as an option.
When this actually gets done depends on when we get other, more important, development items such as an easy-to-use data importer completed.
