Postgres. The main reason I enjoy Postgres is because it is more standards compliant than MySQL, which makes it a bit easier to find documentation because it doesn't need to be specific to Postgres and easier to port to other standards-compliant RDBMSes.
I have had a much easier time scaling Postgres with master-slave replication (it's only just come out in version 9 though). Also partitioning in Postgres is super-easy.
I don't know how MySQL fairs these days, but Postres is completely transaction ACID compliant.
I find user management much easier in Postgres. You create the user and then specify where they are allowed to connect from in a file. MySQL uses a separate user for each host pattern they want to connect from.
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