The air gets a little thin when we over generalize about these materials and coatings.
Both vary greatly in quality, making the generalizations of little value.
For example, I have some PU nylon from Warmlite and some from The North Face that were both purchased 15-20 years ago. They each weigh 1.6 oz. per sq. yd. with the coating. They are different - the TNF coating is flat, the WL is shiny.
Both have survived to this day many winters in a cold basement without peeling.
I did put two layers of the WL together, coatings facing inward but not bonded, and used it for a floor for many years (3.2 oz/sq/yd!) because I was tired of leaky 70 denier PU coated nylon floors. It was actually lighter than the floor material that came with the tent, and worked very well, but eventually, areas of the inner under me did become damp in real deluges - but no water drops.
On the other hand, I have a Bug Dome floor from Wilderness Experience that is PU coated, don't know the denier, between 1-2 oz/sq/yd, and it never even got damp on the inside for several years before I replaced it with silnylon to save weight.
However, my old Eureka solo tent outer was less than 70 denier, and after 5-7 years the PU coating peeled in places, and even where it remained, heavy wind driven rain (but not your average rain) would penetrate and drip into the vestibule.
Note - about the denier - pretty sure that refers to weight of the fibers, not the fabric. Pretty sure you can have a higher denier, lightened by a less dense weave, and a lower denier, heavier due to a more dense weave. So denier does not = weight.
I've never been tempted to use the better PU coated nylons for floors because I do believe the data that the silnylon is stronger than the same thing PU coated, and super quality silnylon is available for much less than cuben. Also, there is very high HH cuben that weighs no more than the silnylon, at a price of course.
I also have silnylon that is over ten years old that at least appears to be as new as the day it was purchased. But most of what I have, old or new, that has been tested by Roger Caffin, has a quite low HH, not much over 1000mm. There is an awful lot of this junky stuff around. Caveat emptor!
Hope the above illustrates that due to the wide variations in quality we need to be specific about particular fabrics, available from particular sources, or at least used in particular products, when evaluating these materials. If you need further proof of this, look at Richard's many test results.
The ultimate tent material would be flexible polyester, and not sag; would be highly vapor permeable and therefore not need a liner or inner tent; would have a super HH, at least 3000mm; and would be available at reasonable cost.
Since we don't have anything like that, compromise is the order of the day, and that's why tents are made out of silnylon anyways.