The spicy-sweet question has no clear answer. It totally depends on what sort of meat you are BBQing. There is no sauce flavor that works with every BBQ meat. It also depends to a large degree on whether you use smoke in your BBQ, and if you do, what type of wood you use. It also depends on side dishes.
If I had to choose just one BBQ sauce, though, it would probably be a sweet one. Rubs, aggressive smoking, and other measures can make up for the lack of a bold/spicy BBQ sauce better than they can make up for a sweet BBQ sauce.