Actually fishing line/leader pounds is the break testing of the line. A 40lb test can withstand 10lb more before it breaks than 30lb test. Diameter in fishing is important because the thinner the line the less visible, apart from color in some situations. This makes Fluorocarbon line more desirable to fish with than nylon. I’m really not sure density plays any part in the strength to diameter ratio. I’m guessing the chemical properties have a bigger influence.
Years ago nylon/fluorocarbon fishing line/strings would sometimes have intonation issues and the solution would be to flip the string around. I see less of this today and any of the Seaguar should be fine.
John
