I wouldn't use WordPress to create a large e-commerce site but I would use it for a ma and pa bakery.
It also depends on what your level your programming skills are at. Joomla is good but if you don't have the knowledge to dig under the hood and customize it by extended default classes, overriding templates and creating custom plugins then you are going to get frustrated trying to do some high level things.
The major downside to Joomla is a massive lack of documentation for doing backend customizations or even developing plugins. a full list of system events for the 2.5 is almost non-existent not to mention a slew of other things that would be nice to know about.
If you go to the official Joomla wiki most pages relating to the 11.5 (versions 1.6, 1.7, 2.5) platform are blank or not useful.
If you don't know how to program very well and you aren't doing any enterprise level websites then I would go with Wordpress as well.
Wordpress is a great CMS and anyone can learn to use it in no time. It has great features, plugins, themes, and a bunch of other fun stuff. But Joomla is more personalized; it's more for developers and a bit harder to use. What I mean by personalized is that it allows the user to work on even the smallest detail.. it's like the stick version of a car..if that makes any sense:)
Joomla and WordPress both are good and better open source content management system. Many people think that WordPress can be used only for blogging, you can easily configure it for other work. Also, it is very easy to install on server. The people who have enough knowledge about coding also will build the website without any difficulties. Joomla on other hand is best CMS for designers and administrators. The choice of these website depends on the website purpose and knowledge of web development and design.
I would suggest Wordpress for the following reasons: -seo friendly -Easy to learn and use -Huge amount of designs and plugins available and easy to customize. -Responsive designs are available to reduce bounce rate that help in CRO(conversion rate optimization) which ultimate provide max ROI.