Leaning towards the T5O....however what is the difference between the 18 inch Current USA Nova Extreme 2x18 watt T5 HO 10K/460nm and the Current USA Nova Extreme T5 X2 Aquarium Lighting Fixture, 2X18 Watt, 18 inch? I know the first one is about an inch shorter in terms of width but it is 20 dollars more, why?