The trick with the metal bars is to not iron all the way down the bar, just leave a couple inches at the end unironed so they're easier to handle. With longer sewn strips of bias, they're going to be pushed along as you iron anyway. I also wave them in the air to cool them faster. My personal preference is for the metal bars since they are flatter than the plastic ones. Some of the plastic bars have ridges in them, which will transfer to your fabric! Using the thinner metal bars for pressing means that your bias will have a better crease that will hold up to all the handling as you applique.