The model you mention is apparently single layer polycarbonate. The only model shown with twin layer is the 6 x 8. Single layer polycarbonate would use about twice as much heat in the winter vs. twin layer, without other modifications.

I have the 6x8 greenhouse. It's easy to set up, attractive to look at, and stable to the wind, hail, rain and snow loads we get here. However, the single layer poly means that it doesn't hold much heat at night. I do not heat it and it stays about 3 to 6 degrees F warmer than outside at night. This is a very small temperature difference, so I use floating row covers inside for additional heat retention.