No comment about stretch side- I don't have that issue.
The tarp- I like the tarp that has been on the DIY forum recently:
Because seam sealing is a PEST. (especially if you're not working with sil nylon but PU coated nylon- heavy but functional)
One of the things that swung my decision for who I was going to buy a tarp from (besides the fact that it looked awesome, was in Spinn and that the exchange rate became more favourable) was that it came already seam sealed. (THANKYOU!)
But hey- it might be something you turn out to be much better than me at!