I haven't done a lot of testing with the Heatsheets and Thermolite bivvies, but my understanding is that the Heatsheets has better reflectivity because of the material. However, there is no way for moisture to escape, so you get condensation on the inside.
The Thermolite has a vent down by your feet to prevent this. The material is more durable, making it last longer in a survival situation or even reusable as a lightweight bag when doing ultralight backpacking in warmer weather.

Either is greatly augmented by some fill material (fleece sleeping bag, fleece jacket, blankets, etc).