It all really depends on how deep you need to fish. One thing to keep in mind is that a floating line will pull a fly toward the surface. The faster you retrieve the faster this happens. Most anglers overestimate the depth at which they are fishing. It is difficult to swim a fly deeper than half the length of leader and even that usually requires a fairly slow retrieve. On a fairly quick retrieve, even a clouser minnow will run up near the surface.

A sinktip line will help swim a fly deeper, but the floating portion of the line will still cause the sinking portion to rise toward the surface. As with a floating line, the faster you retrieve the fly the less deep it runs. Sink tips can be easier to handle than a full sinking line. If fishing in moving water you can still mend to floating portion of the line to allow allow for a near drag free drift of the fly and sinking portion to let it sink to depth before you start the retrieve.

Full sinking lines are better at swimming a fly and keeping it down and a consistent depth, but are more difficult to manage.

I choose the type of line I am using depending on the techniques I am using, the water I am fishing, and the depth I expect to find fish at.

A sink tip line would be a good complement to your floating line and allow you to reach a lot of fish you might miss otherwise.