This depends on your specific needs.
If you have a large commercial e-Commerce site to build then Magento is for you, and if the client can afford it the Enterprise model is not expensive for what you’re getting.
However, if you are building an e-Commerce site for a small/medium client then I would have to say go with VirtueMart + Joomla.
Both systems will do 90% of the requirements for 90% of businesses.
Magento will do that little bit more, but it is more complex and requires more knowledge.
Both systems also have a large development community.
Performance of the site is subjected to your server, the template itself won’t affect to the performance.