Whether I use a sink-tip or a full sink usually depends on the water depth or how I want to present my fly. Both have their purpose. The sink-tip is great in shallow lakes (ie. Lone) in that you can have a lot of line out (cover more water) and not snag the bottom as much. I use mine as somewhat of a shooting head also. Treat the floating part as your running line, it will throw my Pass Lake streamers nicely. If you want to contol depth close in or just get it down and fish it deep, then the full sink wins out. It is also a quick "poor man's chironomid set up" as Stonefish noted. That's why I carry both.