Lutine,
I believe that hobbled shades will be more energy efficient than a flat Roman just by virtue of the extra folds which can trap cold or hot air. Of course there are many other variables here starting with the initial "weight" of your face fabric as well as the corresponding lining that you choose. For example: the best for energy would be a heavy woven fabric with a thermal lining. The least efficient would be a thin faux silk with a flimsy liner. Of course there are many options in between these extremes. I hope that this is helpful!
Laura