There seems to be an assumption that the only lightweight choice is between Cuben and silnylon. But Cuben comes in different thicknesses of mylar, and the quality of silnylon is all over the lot. Assuming, if you can, comparison of the Cuben and silnylon materials that are the best and most suitable for a particular use, there are a host of pluses and minuses for each that have been discussed at length on this and many other threads. The feeling seems to be that these can be weighed, the trade-offs considered, and a decision made.
However, that ignores that there may be more choices to be considered, and I'm not thinking of sailcloth with a 'hard' finish. There is no reason why low denier Dupont 6,6 nylon cannot be given a softer finish coat that is highly water resistant and will also resist sag, nor should polyesters that do not sag be ruled out just because they are not quite as strong as nylon. Nor is it impossible for more high tech fibers to be woven into fabric.
So there may well be excellent materials available or in development that will be superior to both Cuben and the current 30D silnylon for outdoor use. Keep this in mind before laying out a bundle for a Cuben item, particularly one that will be more subject to abrasion and/or elongation of stitch holes in seams. Moreover, there are silnylons already currently available, like that sold by Thru-Hiker, that are much more water resistant, sag less, and can be incorporated in tent designs that make it easy to tighten the canopy and/or are framed in a way to reduce the effects of sag on the occupant.