Fortunately, using a fixed length line system is pretty straight forward; a stick, a string, and a fly.
Probably the most commonly asked question when it comes to getting outfitted to start fishing with a fixed length line rod is – What kind of line should I use?
One thing that is useful in helping to make that decision is to keep in mind how different line designs and materials perform their assigned task.
The purpose of all tenkara lines is to transfer the energy generated by the rod to the tippet and ultimately to the attached fly. Different line designs use different methods to accomplish this transfer.
Typically, a single strand line relies on the inherent stiffness of the material to transfer line energy. So in effect, it works like a bow and arrow. The energy generated by the rod creates a spring effect in the line, energy bends the spring, and the spring releases the energy when it straightens out. That in part is why flouro is generally a preferred material over mono for tenkara lines. Flouro as a material tends to have more inherent stiffness than does mono.
A furled line on the other hand does not rely so much on stiffness; rather it relies on its own inherent mass. This is typically how the fly lines on reeled rods transfer energy. That's why it so important to get the right line size for a given reeled rod. Sufficient mass is required to capture and transfer the energy supplied by the casting motion of the rod. A furled line may have more mass than a single strand line, but it does not have to be as stiff.
So to greatly simplify, a single strand line can be relatively thin (diameter), but needs to have some level of stiffness. A furled line doesn't require stiffness, but it needs more mass which usually equates to a larger line diameter.
So where the design and material properties comes into play is how the line reacts when it does (or does not) come in contact with the water. A single strand line is thin, so it exposes less cross-sectional area to currents, but when a segment of the line is moved by a cross current, the stiffness causes that movement to propagate thru much of the rest of line.
On the other hand, since a furled line can be fairly limp, a segment can be moved by a current and the line is limp enough to cause a bend in the line, rather than dragging the rest of the line along.
Similar considerations come into play when the angler is interested in minimizing contact between line and water by suspending the line over the water. It is easier to suspend a stiffer, lighter line (single strand), than a heavier, limper furled line.
There are other considerations, such as when nymphing, a thin single strand more easily cuts thru the water, but again the stiffness may be a factor in influencing the movement of the fly.
Bottom line, there are tradeoffs based on the materials and designs that are used. The best line choice often comes down to where and how you plan to fish.