The 100% polyester thread is important for gear which would represent a hazard if it fails. The poly/cotton blend is subject to weather damage and rot in a way 100% polyester is not. So for hammocks and structural items which my health and safety depend on I would use only 100% polyester. For stuff sacks and sleep gear and even for tarps.. I am much less picky. My wife was a professional seamstress and as such we have a ton of dressmakers thread and supplies. Much of the thread is poly/cotton and for most practical applications where safety is not a concern I see no problem using the blended thread. As always, YMMV and if you are going to buy only one big spool make it 100% polyester. If on the other hand you have a stockpile of dressmakers thread at your disposal, do not hesitate to use your stock pile to make the practice pieces. Even hammocks that will be used sheltered from the weather would be completely safe with blended thread... at least IMO.