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Either one will work. The difference is that propylene in the yellow cylinder burns hotter. It has a flame temperature of 3,600 degrees F opposed to 3,450 degrees F for propane. A hotter flame means it can heat the pipe faster so it is ready to solder sooner. The higher temperature can also overcome the effects of a small amount of water left in the line, which is common in plumbing retrofits. The negative of propylene is the cost: about three times that of propane.