I wouldn't use the heat pens. The manufacturer has said they are not meant for fabric. They have 2 components: The thermal ink, which dissipates with heat, and gel ink, which remains. Many have had them "ghost" after use. On lighter fabrics, typically a light brown line. On darker fabrics, it can show as a white line or as what looks like a grease mark.
Whatever method you use, unless it's a Herra marker, I'd check it with the fabric being used every single time. It seems like every pen out there has occasionally caused problems. I don't know if that's a reaction with the specific fabric or how the fabric was prepared, but to me, it would be worth the inconvenience to avoid a more serious problem later.