Technically, the 'silnylon' fabric is 'double-coated'. That is, a coating of silicone polyer has been applied to both sides. Well, fair enough, but there is a huge difference in the result between PU and silicone coating. The PU coating sits on the surface of the fabric, but the silicone polymer goes right in. As far as I can see (with a microscope) the silicone polymer completely permeates the fabric fibers and forms a layer right through the fabric. As far as the final fabric properties are concerned, you should not think of 'silnylon' as a 'coated fabric', but rather as 'nylon-fabric-reinforced silicone polymer sheet'.
This is a bit like fibreglass or glass-reinforced epoxy.
This difference translates into mechanical properties too. A key parameter is 'tear strength'. It is claimed that a PU coating focuses the stress in a tear right at the tip of the tear, and this actually makes a PU-coated fabric behave weaker than the base fabric. However, the elastic silicone polymer in silnylon fabric takes over and distributes the stress across a number of threads, and this makes the silnylon fabric significantly stronger in tear than the base fabric
- reportedly up to 2.2 times stronger. Note this does not apply to EPIC fabrics.
Silicone is the stuff that is used to seal aquariums, bathtubs and sinks, and other applications where water must not be allowed to leak through. When it rains on silnylon fabric all the water stays on the surface, and a quick shake will get rid of almost all of it, and its associated weight. A few minutes in the wind or sunshine and silnylon will completely dry out. Certainly, I have found that I can shake most of the water off my silnylon tents, and the packed weight is rarely much different. In comparison with PU, silicone is 'hydrophobic' (water hating). By its nature it repels water, and water will not act as a solvent on it. Your packed-up wet tent may get mouldy if you leave it long enough, but it won't go sticky. Finally, the silicone in silnylon completely permeates the fabric fibers - it can't peel off.
But remember: it does not breathe!