PostgreSQL vs MySQL benchmarks when a 2nd level cache is included
Left unconfigured, PostgreSQL will outperform MySQL. In fact, left unconfigured, InnoDB in some older versions of MySQL are better than newer versions of MySQL.

Tuning PostgreSQL and MySQL and then Benchmarking for occassional usage in the presence of memcached is a lot like building two cars, drag racing them, and then parking them forever. That only leaves the open road and whatever happens to be in the middle of highway.

If you have a large working set and high turn around of data, then perhaps memcached and other second-tier caches woiuld be unnecessary. From that point forward, choice of database and benchmarking would be worthwhile and intriguing.