Joomla and Drupal have a few differences but are very similar in a lot of ways too.
Drupal has support for multiple sites as well as a forum type feature and a few other things. Joomla has plenty of extensions and some may say a little easier to work with. But really both are very good options, it just depends on what you're comfortable with using.
One thing I've learned about all the Open Source goodies out there (Wordpress, Joomla!, Drupal, Magento, Refinery) is they all usually have their underlying purpose. I do most of my work in Joomla! and then Refinery, but personally if someone came to me with a need for a site solution primarily geared for e-commerce, I would not select Joomla! with an add-on like VirtueMart, I would select Magento (Or Joomla! with RokCart only for very simple carts). So basically I think what I am trying to say is that there really is no clear "winner" in the suites, no one suite is better than the others, they all have their places and if they did not they'd go the route of PHP-Nuke.
The best bet is to pick one to participate with (i.e. I like to participate and give back with Joomla!) so that you can be intimate with at least one suite, and then just be a great coder so that you can tweak the others to clients' specs if the need arises.
Hope that helps! And even though it is not free (it is Open Source) JomSocial I feel is a superior suite for building social communities. Also look into K2 if you already haven't, you'd be surprised how community-oriented that can make any site.
When someone is impatient and says, "I haven't got all day," I always wonder, How can that be? How can you not have all day?
Thanks Jeff for your answer.
You gave me homework: to learn about Refinery.
I'm using Jomsocial, but I don't love it. It's hard to translate the e-mails, and you have to redo it after every upgrade.
Maybe the new community framework included in last template, or someone will come out with a new community platform. I think the key here is to be compatible with other platforms (phpBB, magento...) How hard this could be? A lot?