I am looking to build a somewhat complex site, and got offers both to build it in Drupal, Joomla and custom PHP. How can I determine which is better? I want to be able to add and change things later on.
If you are planning on making changes later then perhaps one of the platforms would be the better option since they are continually being updated and improved (including security updates) whereas if you opt for custom php then you are pretty much stuck with what you get with a custom script. But it all depends what you plan on doing with it.