The two are pretty much the same in context. A whole host of prequalified joints, alloys, and fillers are available and prequalified processes that include GTAW which is not the case with D1.1. Preqaulified thickness are .062" and up just like D1.1.
Although I purchase SWPSs from the bookstore for my D17.1 work, and apply them to my D1.6 requirements. My customers don't get the prequalified business and feel more comfortable with official documentation.