They both matter. You could have an insanely high lumens/watt but if you only have a 1 watt light then you aint got enough. If the bulbs were the same the two would put out about the same amount of light, but since the T5 is more effecient it draws less power to put out the same amount of light. And like everyone always says lumens arent a good measure of light for plants, so dont make any big decisions based on lumens.It's not the wattage that you should be concerned about it's the lumens per watt. The T5 gives out more lpw than the T12 for sure.