As far as SEO goes both urls you listed will be optimized the same but I can see what your saying from a user friendly standpoint if you are giving the url to your users. The issue is that the productdetails page reads the productid in the url to so we would need at least the productid in the querystring. The cart uses the dnn navigateurl function to move between user controls and that is the url that is generated. Do you have friendly urls in your portal or have you tried url master?
I do have friendly URL's set up in the portal and I was trying to build a rule to have the friendly URL created from the long URL for the product page, but I was not having too much luck, as it is a very complex language. I have not tried URL master, but I will check it out.
That actually brings up another question, will I be able to link directly to the product pages within Smith Cart as a submenu item under the main menu item "Products" that we currently have? We would prefer to not have to maintain 2 pages for each product, one from Smith Cart and a second one that we would have to build under the menu item "Products" that would have a link to Smith Cart for purchase.