Here is my 0.02 cents,
The big difference is on the design. There is a give and take in here.
In MODx, you have greater control over design, but you cannot get all the fancy features such as blog and bla2 to be setup easily.
In Joomla, you have almost no control over design (unless you’re a css master), but you have all those fancy ceatures available that you can easily installed them on your site.
So if you’re a designer or advanced user, I might suggest you to try out MODx, but if you just want to get your site up and running without having to bother taking care of the design and etc, then go for Joomla.
In the end, it’s all depends on your needs. But believe me, in the next few months, we will have more features/addons added to MODx, so you can just easily install and use them