I believe the density compensating lines you are refering to are a WF line normally used for stillwaters. I have used them for years and just purchased the Orvis DC line for a trip to British Columbia for brook trout on Fortress Lake. They are very popular in the stillwater scene. The 350 grain depth charge lines, I believe, are a shooting taper. The head is 350 grains and will likely be attached to an intermediate running line. They will load and cast much further than the DC line.
The DC lines are used in freshwater to prevent the hinge that used to form in the older sinking lines. For stillwater applications the takes are sometimes very soft and the angler must be in direct contact with the fly.
"Twenty years from now you will be more disappointed by the things you didn't do than by the things you did. So throw off the bowlines. Explore. Dream. Discover." - Mark Twain, 1879