Tony,
I can't tell you anything about the conference (or Norman Freund

), but I can tell you that you are making the correct decision. Do not accept everything you hear to be 100% true and question what people say asking for proof to back it up.
I don't believe Joomla! is unstable and I think it's future is only looking better as more and more people start using it, so I wouldn't worry about it's future being limited.
If they are pessimistic about the future of MySQL, I would like to hear the alternatives they are looking at for a database. There are others out there, but I don't see MySQL going away any time soon. Are they worried that it's quality is going to slip or something? Also, doesn't Drupal (it sounds like that may be the CMS they are backing) use MySQL as well?
One CMS is not
always better than the other. I think they all have their own purpose and some are better suited for certain tasks. I would look at the requirements your team has and try to make your decision based on that.
Make sure that the other Committee members understand that you are not questioning that they heard this at the conference, but you think they should be questioning the people they talked to at the conference. Anyone that tells you they prefer one CMS over another for reasons A, B, and C is giving you their opinion and trying to help you make an informed decision. Anyone that tells you one CMS is always better than another and provides no facts to back it up should definitely be questioned.
If you have any specific questions that you want answered that will help you make your decision, feel free to ask. If we can address your teams concerns, it will help them make an informed decision. I don't think you will be making a bad decision going with either CMS. I recommend Joomla! to most of my users because I (personal preference

) think it's easier for someone with less experience to use than Drupal. There are other factors to think about, but that is usually a big selling point for my customers.
By the way, pat yourself on the back for me for questioning the decision rather than just going with the masses.