Joomla is a CMS, 'suitable' for blogs and websites that frequently update their content. Magento is a well known so called 'ecommerce solution' aimed at people who need a webshop rather than a casual website or a blog. Although you can make webshop with Joomla as well, but you do need extensions for that. What is it you exaclty need? Maybe there are better options than those 2.
Joomla is a CMS (Content Management System), Read More about content management system here... http://en.wikipedia.org/wiki/Content_management_system Whereas Magento is designed purely for ecommerce solutions such as shopping carts etc... You will have a better idea when you will see other shopping cart solutions like bigcommerce, opencart, prestashop etc...
Joomla is not an ecommerce but it can be with installing a shopping cart extension. IF you are going for an ecommerce website better use magento. If you are going for your business website and selling a couple products on it its ok to use joomla
If you want to make blog and website that frequently update their content then you can use Joomla. Joomla is CMS(Content Managment System). If you need to use ecommerce in your website then you can use Magento. Magento is ecommerce concept.
I would prefer a full fledged e commerce platform than using an add on on any platform. How is OpenCart and PrestaShop ?
Can you please specify your requirements so that we can guide you in right direction, are you looking to open up online shop?
Unfortunately said blog system does about as good a job of it as the mods/plugins/extensions/whatever they're calling them this week for turdpress and joomla do of turning those into an e-commerce package; which is to say pissing all over the place with ineptitude; sometimes you're just really best off using the right tool for the right job, instead of trying to use one tool for everything. You can drive a screw with a hammer, doesn't mean it's gonna hold.