Lighting: How to know which is more intense/bright?

What reading should I use? Wattage? # of LEDs? PAR? Lumens?

Looking into a small 12" bar light. I have an aqueon freshwater light and an aqueon 12-18" optibright light. Though the optibright has more LED and is 15w, it looks dimmer than the clip on with less LED and wattage.