A: To some degree the reel used will be the deciding factor. If the spool has little line capacity, then thin 50-pound braided (gelspun) might be the best choice. You certainly don't need more than 150 yards of backing. Add to that a 30-yard fly line and you have nearly enough line to stretch to the end of two football fields. Despite all the hype about the modern braided lines for backing, most fly-fishermen still use Dacron except for offshore work. I recommend 30-pound-test Dacron. It wears well and will provide years of use unless abused. However, 20-pound-test Dacron does not wear as well, and if used hard it should be checked routinely and replaced if wear is evident.