Difference with Commerce Expresss Checkout

There is one less click. Express Checkout adds a button to the node that redirects the user to a checkout page with the product added. Single Page Checkout loads the checkout form onto the node and adds the referenced product so you don't need to click an extra button to get to the checkout form.

One more follow-up: I built a couple of D6 + Ubercart sites that did similar to Express Checkout, where you would have one URL to view the product and one URL to redirect the user to the cart with the product added. Clients were always confused by this and requested to have a single URL that they could add to marketing materials, and also a URL they could customize. Because the checkout form is on the node, this allows for both a single URL and for customization over the URL.