Silpoly is not necessarily lighter...the 30d silpoly the next batch of our camo is made from will be about 1.35 osy, current 30d silnylon is 1.3 osy, and current 20d silpoly is right at 1.2 osy. Then you have some companies offering silnylon in a 20d version. My 20d silpoly is heavier than RSBTR's 20d silpoly which they advertise as 1.1 osy...So the weights have nothing to do with the material and everything to do with a combo of denier, threadcount, and coating weight.

In general nylon has a higher tear and tensile(breaking) strength than the same weight polyester simply because nylon is all-around stronger pound for pound than polyester is. Poly will stretch less (when soaked or otherwise) and it will absorb less water weight. Poly still stretches some, just not as much as nylon. The extra stretch of nylon adds to it's strength though, in that it acts as a built in shock absorber during a heavy wind gust, so with poly stretching less in that situation, in theory the nylon tarp would have a lower wind load due to extra stretch. Is the extra stretch and breaking strength needed? Only time will tell, but other companies have been using 20d silpoly for awhile with what seem like good results.

I can really only speak for our 30d silnylon, we've had excellent long term durability results from that. We've just started using poly and we're not even certain at this point if we'll be sticking with a 30d or 20d poly, we'll eventually decide based on how well the 20d seems to hold up. The 30d silnylon is still stronger than the slightly heavier 30d poly though.

Ploy is said to have better UV resistance, but don't think you can treat your tarp any differently than before, I had a polyester tarp outside for about 2 months and the fabric lost ALOT of strength from the sun exposure, so you still want to avoid things like that regardless of whether your tarp is poly or nylon