Drupal, Joomla, Mambo, E107, Dragonfly CMS, Limbo, phpBB, SMF and WordPress. I have used them all. In terms of upgrades, I can tell you right away that WordPress is the best in every way. No hassles, no migration scripts to install, over write your existing installation and presto, it will prompt you to upgrade and everything is done in a breeze, no sweat. Of course, it is always a good measure to backup your installation before upgrading. This will provide you a fall back in case disaster happens.
Others may have a diffrent view on this but this is what I actually experienced. Migrating from Drupal 4.X to 5.X or 6.X is a pain, much more with Joomla 1.X to 1.5. That is why, these heavy weight maintains two versions of their softwares. why have they not integrated a software script in their system that when a new version is installed, database will automatically be upgraded. If the database structure is different, then new fields will be filled up with default values.
If you want to be online and want to maintain a simple site with simple features, go for WordPress as this will make you more future proof.