I just went through this between coton and poly. I chose cotton but no longer buying Mettler. I've just developed a problem with Mettler creating too much lint. I'm switching to Aurafil. Your needle question you may find an answer to at the Schmetz site. Google Schmetz and you will find needle charts telling which to use for thread size and weight and purpose. Don't be shy about posting. Being a "super user" can sound intimidating but everyone who has responded to my questions has been great. I"m new to the Board and posting, but we've all asked these questions and this Board is the best place to be for answers, knowledge you didn't even know you needed and inspiration. Welcome