ranking is not a function of which cart you use, its how well a site is designed and inbound links. as long as the cart lets you design the site however you want around it, its the backend features that matter the most.
Without being able to rank the pages, you will have to build a second set of pages just to rank. I want a cart that can be ranked in the search engines. Why would I want to change carts to loose so many #1 spots??