All soft tubing has some plasticizer content, which over time has a tendency to leach out and cloud the tube. Some tubing is worse than others and you sometimes see little rings of accumulated plasticizer accumulated in dead flow spaces like at a vertical barb fitting.
The black rubbery looking tubing like ZMT or norprene supposedly have no plasticizer, but the flat black aesthetic only works well with some builds.
Hard tubing has no plasticizer and comes in clear and all the colors you might need to accent your build.
With modern day pumps, the added fittings on a hard tube build, even one done more with fittings than bends, isn't enough of a flow issue to not go the hard tube route.
Hard tube builds are a bit more costly, and definitely more time intensive in running the loop, but the freedom from plasticizer worries and having much longer intervals between maintenance teardowns is worth it to most experienced builders.
Oh, and did I mention that it looks really cool too . . . .
Edited by IT Diva - 2/12/16 at 2:28am