This is an older thread, so I don't know if the question is still relevant for the OP - but others may find this useful. It's been over four years since I evaluated these two in-depth, and I ended up going with a different CMS tool altogether...but I'll give my impressions based on what I remember, what I've observed since then.

I've read horror stories about upgrading / maintaining both of these apps - so you'd probably need to try them out, see which one you're more comfortable (or least -=un=-comfortable) with, from an administrative perspective.

Both seem to have active user communities - which can be good, especially if you're looking for alternate solutions or help getting XYZ to work.

Both are free and open source. Both are extensible, and have an active user and developer community. Both are apparently cranky&crotchety when it comes to upgrading - the process seems to have more than its share of bumps, from what I've read and what my clients have experienced (and asked for assistance with). WordPress is a slight bit easier to configure than Joomla, from my memory. However, Joomla seems to have more in the way of CMS-like tools built in. WP is more focussed on the small, "blog"-like site - something with registered users that wants to add pages, but doesn't necessarily want to do things like polls, article ratings, file management, link management, banner / campaign management without installing third-party plugins / extensions. You may decide to choose one tool over the other based on your plans for the site.