PostgreSQL versus MySQL for EAV structures storage
Both excel at a simple task like this. If you end up having big queries where you search for entities that share many attributes ("relational division"), I would expect PostgreSQL at an advantage for its superior index handling.

In particular, multiple joins can be combined with bitmap index scans - a feature that is not present in MySQL. It has an "index_merge" feature to substitute for that.

For PostgreSQL you may also be interested in the hstore extension for storing sets of key/value pairs.