Likely either or. Wordpress is much better if you can cobble it together to suit your needs. The code base is alot better. I had to recover 7 different sites for people this week that had their joomla site hacked. Nigel
I would say it is Word Press, initially dominant in the blogging arena, it is increasing common to see it as an CMS given its easy use and amazing array of free templates and plugins.
wordpress is technically not a CMS but it is increasingly being adapted as one. still, a bit hard due to inherent commenting system to be turned off, etc. if you want a regular site. on the other side, joomla is a nightmare to blog in but CMS wise, it is my favorite.
since you said or blogging system. It's definately wordpress by far..like someone said it can be used as both.