Great summary Blancastella. I think it illustrates my point well, use WordPress as the CMS and Magento as the payment portal on a site that is already using it. In that case the investments have been made in components and in learning the interface. It is unlikely that any of the existing plugins will do what it can for commerce.
However, I have only moved one client to Magento or any third party cart from WordPress. They were building a full online shop. They have a brick and mortar store with tons of products and wanted to set all of that up with inventory tracking that integrated with the system they used in the store. Magento had a module for that and any plugin would have required my to write some kind of gateway to try and merge the system.
Every other client was able to use a WordPress plugin. I've used Shopp, Cart-66, WP eCommerce, Jigoshop, WooCommerce, and a few others. Each required some custom work to get things exactly like the clients wanted, but it was much easier than setting up a standalone cart and teaching them a whole new system.
Still, there are times a full cart is the best choice. When that system is already in place or when the client needs far outreach what the available plugins can offer.