I've used both silk and Bottom Line, which is a 60 wt poly. They both blend and sink into the fabric beautifully, and I have had no problems with the Bottom Line melting. However, I prefer the Bottom Line because I can buy assorted colors on prewound bobbins, which is much more affordable than buying multiple colors of silk on spools. If somebody ever came out with an affordable assortment of silk on bobbins, I could be persuaded to go back to silk.