<p>I'm having to start building the architecture for a database project but i really don't know the differences between the engines.</p>

<p>Anyone can explain whats the pros and bads of each of these three engines? We'll have to choose one of them and the only thing I actually know about them is this:</p>

<ul>
<li>Mysql &amp; Postgres:

<ul>
<li>Are free but not so good as oracle</li>
<li>Mysql as security problems (is this true?)</li>
</ul></li>
<li>Oracle:

<ul>
<li>Best data base engine in the world</li>
<li>Expensive</li>
</ul></li>
</ul>

<p>Can someone clear out other differences between them? This is a medium/large (we're thinking of around some 100 to 200 tables) project with low budget, what would you choose? And with a higher budget?</p>

See the comparison tables on wikipedia:
http://en.wikipedia.org/wiki/Comparison_of_object-relational_database_management_systems
&&
http://en.wikipedia.org/wiki/Comparison_of_relational_database_management_systems
Oracle may or may not be the best. It's expensive, but that doesn't mean best.
Have you looked at DB2? Sybase? Teradata? MS SQL?