I agree about light diffusion but not about heat holding capacity, glass is thicker than polythene and I have always found a greenhouse slightly warmer than neighbouring polytunnels, on cold nights.
I also notice, on days of bright sun followed by black clouds, how quickly the temperature drops in a polytunnel, and mine has plenty of air capacity (18 x 30 feet). Towards the end of a fine day it is different because the ground has warmed enough to hold heat, whether it is tunnel or greenhouse.