"MySQL 5.0 is in beta, and has been feature-frozen since April. Back in 4.1, its abstracted table-type has been put to advantage with odd engines like Archive (only insert, no update); Blackhole for fast replication; and an improvement to MyISAM for logging (allowing concurrent selects with inserts-at-table-end). Their Connector/MXJ lets you run a native MySQL server embedded inside a Java application. In 5.0 we're seeing stored procedures per the SQL:2003 standard, triggers, updatable views, XA (distribution transaction), SAP R/3 compatible server side cursors, fast precision math, a federated storage engine, a greedy optimizer for better handling of many-table joins, and an optional 'strict mode' to turn some of MySQL's friendly nonstandard warnings into compliant errors. And they're working on partitioning, ODBC, and letting MySQL Cluster's non-indexed columns to be stored on disk.
PostgreSQL 8.1 is expected to be released in November or December, after a feature-freeze in July -- and it's an impressive list of new features. Their optimizer will make use of multiple indexes when appropriate, which is pretty darn exciting. The recommendation will be that in most cases it will be most efficient to have only single-column indexes and let the optimizer figure out which combination to use. They're implementing a 2-phase commit, they're bringing the automatic vacuum into the core code, and they removed a global shared buffer lock so they're now getting 'almost linear' SMP performance scaling. I've never felt the need for Postgres, but I'm definitely going to look at 8.1."
It seems like these databases are changing versions really fast. Wasn't 4.1 in beta until just a few months ago?