I think the Q-power-600 that Think uses is far superior for all the reasons stated by the other posters. So much so that, after several years of use, I intend to replace my Fenn stainless steel lines with it. Why?
"Stainless" steel is somewhat of a misnomer in that even stainless steel can suffer from some corrosion, albeit much much more slowly than other metals. The twisted stainless cables can also suffer corrosion inside the twist, mostly invisible to the user. Flushing freshwater over saltwater bathed gear, like the rudder lines, can only so so much. Unless you can physically wipe all the surface (not likely to be able to do that inside a cable) or apply some chemical de-salter, some of the salt remains.
Your argument about the sand acting as an abrasive may have merit, but I think it would apply equally to the steel lines; and, the action of the individual steel threads rubbing each other may also degrade the line (as it may indeed also do to the synthetics).
As the others have said, the Q-power line is so much more easier to work with than using dyneema or steel cables. Because of that (human nature being what it is), you would likely be more inclined to replace your lines after an appropriate time instead of "just going a bit longer with the old cable" (and having it break at an inopportune time). UV degradation is a always a concern with synthetics (but not steel), but that is why the strong core of the Q-power line is covered by
braided polyester.
I have been using the Q-power and replacing other lines with it, as they come due, for over ten years, with never any issues. If line stretch is a concern vs steel, I haven't noticed it, or at least not enough to care about it.
Generally, one uses steel when you need the strength, but then you must carefully evaluate for corrosion, with much of it perhaps hidden from view and affected by numerous factors. One generally uses synthetics to save weight, bypass the whole corrosion issue, and for durability, UV degradation notwithstanding. IMHO, corrosion of metal is much more complex to deal with; with synthetics and UV, if you cover the susceptible part (often with a less-UV-susceptible synthetic like polyester), you have effectively dealt with the problem.
The more I learn about it (in the context of my career or personal life) I have come to deplore corrosion, and whether due to dissimilar metals, water, and/or salt, it is insidious and always occurring.
Bill L