flexwatt is a thinner, wider, flatter heating source that requires some electrical wiring.

the heat rope is simply a long cord that plugs in on one end and run it where ever you want.

Flexwatt works great. Probably the most popular.
the heat rope, you will probably get mixed reviews on. Personally, I like it. It depends on how you're using it. Vision Racks are made to use the heat rope.