RE:What is LC-13 line?
Other than price, the trade off is in the casting ease. The T-series, QD (quick descent), depth charge, yada yada are all based on the concepts pioneered by the Harry Lemires and Wes Drains and other Washington state steelhead junkies who were by the way the earliest flyfishermen to wear stripping baskets as far as my research would show. They combined sinking heads with shooting lines of all types - flat beam, braided, mono and floating level line; and eventually combined half floating / half sinking to pioneer the sinktip lines. Nowhere do these lines come into their own like a steelhead river. Anyway, I digress.
The hi-D fly lines are actually tapered lines, skinny on the ends and fat in the middle. This provides some facsimile of loop generation in the cast, as the Cortland 444 QD325's widespread popularity proves. The LC-13 is a true lead core line, level and rigid - yet under an energized cast it becomes flexible enough to turn over, kind of. Summary: lines cast better than LC-13.
The advantage for LC-13 is depth penetration even in fast deep currents. The thin profile and real-lead density can't be matched by impregnated coating technologies. For get-down and dirty efficiency, nothing beats it. Summary: LC-13 gets down faster and further than hi-D lines.
There is a place for both. LC-13 is great to have on a hard rip off Gorilla or Bearses off Monomoy; or the Middle ground, etc. I wouldn't use it for any situation where I really had to cast with grace, accuracy or distance.
Hope this helps,