A: Choosing a curtain will depend on a few things, namely, where do you want to put them, what is the size of the windows and what colours do you have in the room.
One of the double curtains is semi-transparent (shear) The role of shear curtain is to have adequate visibility during day time and the thicker curtain is used for privacy or to cut off light during the night. The use of the double curtain will depend upon your utility for it.
Roman curtains are generally preferred for smaller size windows as well as windows that have a box frame. The box framed windows are accentuated by the roman styled curtains and they give it a photo frame look.