We make our HD Mainline tubing out of high density polymer, and our 30P Mainline tubing out of low density polymer. For reference, the HD Mainline pipe is a darker blue color, while the 30P Mainline pipe is light blue.
Due to the low density polymer in the 30P Mainline tubing, it's more flexible and more forgiving than the HD Mainline. It's also heavier to put in the woods, but it tends to stay a lot cleaner than the HD.
Compared to the 30P Mainline, the HD Mainline is a harder, stiffer pipe with a very smooth inside wall.
One last important difference is the expansion and contraction coefficient of the HD Mainline pipe versus the 30P Mainline pipe. The expansion and contraction coefficient tells you how much a pipe expands and contracts when you heat it and cool it. For the HD Mainline, the expansion and contraction coefficient is much higher than the 30P Mainline. This means that the 30P Mainline is a lot more stable.