I would speculate that it is more than just the material that causes stretch. I have the Moment and I think the shorter length of material from the center arch pole to the ends minimizes sag due to the stretch. A few years ago I bought a Cloudburst and the length of it from the arch pole to the end was very, very long (I felt too long). I returned this tent and got the Moment this year. I would think the Rainbow design would have the least sag just due to its pole design. Hey, I am no mechanical engineer, so I may be entriely wrong about this! I for one, am willing to re-tighten a few more times for the reduced total price. $100 more is a significant factor for my budget. But I do agree that it would be nice if both fabric options were available. By the way, all my tents including coated nylon tents sag when wet. I guess I just figure tightening is part of backpacking!

Another factor is wind. Repeated wind movement of the tent fabric can loosen or strecth your ties or stakes. I also would be careful on the tie-strength. If the ties are static (no stretch) the wind can add lots of load and break very small ties. I also find very small diameter ties simply difficult to work with - I prefer larger diameter ties even though they weigh slightly more.