You can’t create custom post-formats. What you could either do is have a post category called portfolio and then use the body/post class specific to that to control the display on the front end, as this is what post-formats are designed for, or go for a custom post type. Personally I go by the idea that, if it makes the posting screen less complicated for the client to work with for a specific post type, then a custom post type is the best option.
You can still include custom post types in your standard loop – http://justintadlock.com/archives/2010/02/02/showing-custom-post-types-on-your-home-blog-page so you wouldn’t lose that chronological order, it would just make it easier for the client to add portfolio posts, if they are to be so different to normal posts.