It depends on your application, exposure to sun and projects goals. If reducing energy bills is one of your goals, make sure the windows are energy star rated, with a U-Factor below 0.30 and Solar Heat Gain Coefficient below 0.25. This ensures the insulated glass unit will block out the heat or retain the heat but not the sunlight. Thicker glass such as ⅛” is also helpful for durability and noise reduction.