There are a number of good, objective, and informative comparisons available on the Internet, such as http://www.databasejournal.com/sqletc/article.php/3486596. You can Google something like "Postgresql mysql firebird comparison" to access these. I am migrating an application from Paradox and took quite a while to review the options before settling on PostgreSQL. I am in the early stages of the migration and so far, I have not been disappointed.