Never played with heat insulation, only sound insulation. What color is your paint job. If it is white you may be able to get away with the cheaper stuff, if black you must buy the best. Look at how high of temperature the adhesive that holds it in place can handle. A black car can get over 200F on a nice sunny still air day. The adhesive needs to be able to handle that and still hold the sound deadening insulation when vibrated. Sound Deadener Showdown has a good writeup on this.

-- Bryan