I have used Joomla a while back and I am not on prestashop. We looked at magento, heard it is decent but really heavy/big machine. Because we are small I was advised not too use it. Not heard of big commerce, we also considered zen cart and os commerce. Now we even considered wordpress+ wooshop and other ecommerce themes.
Ended up using prestashop back when it was quite new. It is not perfect, support sucks, forum community not that great. But as much as I hate it, it sounds like the other platforms are worse in some areas. Overall they all seem to have their down side. I think prestashop is not great for seo. The good news is most of my small competitors use it, so I am not at a disadvantage. Also a lot of their issues were fixed in new versions, and I am still behind so can't really complain. You would not be able to do what you want with default version of prestashop. They would add a number like this: domain.com/7-categroy name/199-product name you should be able to code it manually though.