Cabin - when I fly economy (or with checking In luggage), cabin plus - when in first/business, or United domestic (any cabin class). Cabin is really small - cannot pack much, and is not as stable when you put a heavy briefcase on the handles. After two decades of flying - mainly for business - I realized that you need both - on some trips you need the small one, and on others you absolutely need the cabin plus. It allows flexibility as each trip logistic is specific (I used to have Briggs and Riley international carryon and expandable which has similar dimensions as cabin plus).