Artificial sinew can be made out of nylon or sometimes polyester. I don't think the manufacturers in India or Mexico care what material they use. If nylon then its too strechy, if polyester, then its the same stuff as dacron. Trouble is the package isn't always lableled, so is a guessing game.
Considering the price of a roll of artificial sinew and a roll of b-50 is about the same, get the b50.
Artificial sinew is great for wrapping fletchings as well as stitching bowhandles, knife sheaths etc. It won't go to waste.