The higher the pound test,the thicker the line.The diameter of your line will affect the height of your lures in the water.The thicker the line,the higher the lures will run.Most of the best, most experienced trollers actually use 10# or 12# test mono.The reason they use that low test mono is because most of the dive curve charts use a 10# diameter mono when making those charts.You could still use 20# line but i think 15# is best for a beginner to trolling.I really have never had our line break,so 15#is plenty strong enough.The thicker line also helps with releases holding onto the line and its easier to get your lines untangled.