I used to use MySQL quite a bit, and lately MySQL has actually been really good in terms of performance and feature-set. Both MySQL and PostgreSQL have their advantages, and as long as you know when to scale and shard your database it's all relative in the end regardless of which of the two you choose, both can be tuned to do the same things.
PostgreSQL has traditionally focused on reliability, data-integrity and integrated developer-focused features. It has an extremely sophisticated query planner, which is capable of joining relatively large numbers of tables efficiently.
MySQL, on the other hand, has traditionally focused on read-mostly web apps, usually written in PHP, where the principal concern is with optimizing simple queries. IMHO, MySQL is relatively easy to get started with, and has a great manual for beginners. Not to mention that it is easy to install it.
If all you are doing is reading single rows from a single table using an index, then perhaps PostgreSQL might be an overkill and MySQL may perform better in this case. But then again, why would someone with this usage use a RDBMS? Shouldn't SQLite be sufficient? Or an Excel sheet? :-)
That said, if you are going to start a new DB application project, then use whatever database that you are comfortable with and makes it possible for you to get the project done sooner rather then later. In my case, I normally use PostgreSQL when developing Python/Django apps due to the ease of integration with Django itself, and use MySQL when developing PHP-based sites since it is readily available on most hosting sites for my clients. I do also use Java with MySQL as well.
P.S.: I am not sure if Wordpress works with PostgreSQL out-of-the-box. Anyone?
Regards,
Frankline