.....

You need to escalate this to the owners of the company. Before then though, you need to get all your ducks in a row:
1) Do your research on all available CMS tools, as it relates to all the features the company needs
2) You need to weigh #1 against the back drop of whatever you are most comfortable with
3) You also need to weigh #1 against whether or not the owners of the website will be making lots of updates and their skill level for updating stuff

Personally, having dabbled in only joomla and wordpress, I would say err on the side of low learning curve (customer-wise) and go with joomla. wordpress is cool, but as you know, it can get a bit messy, especially for customers.

Regardless, you need to escalate this issue to upper management, stressing to your colleagues that it is nothing personal and that you want what they all want - the overall success of the project.

good luck, and do let us know how it goes.

It seems there is self-ego problem here not job problem, because which ever way (joomle, drupal and build-from-scracht) will do the said job fine and the proceed will be shared equal.