The 18g has 18 2-watt LEDs and HNH says that the 18G is 32 watts. But isn't 18 2-watt bulbs 36 watts? The OPI does have 32 smaller LEDs (don't forget the four lower down on the side, two on either side). I don't know the individual wattage, I'd guess 1, maybe 1.25 watt each, but the lamp says it is 40 watts. Remember that wattage is only a measure of input, not output.
From my experience with these lights, I'd say they are about equal in power and output. The things that differentiate them are less a matter of quality and more of personal preference. The OPI is black all-metal construction that looks a bit like Darth Vader's helmet; the 18g is a compact white plastic with a light shield. The OPI does not have a bottom, just feet to lift it for pedis; the 18g has a magnetic removable mirrored bottom. The OPI has a sensor strip towards the inside back that you touch to turn on - if you remove your hand, it stays on until the end of the cycle; the 18g is motion activated and if you remove your hand, the light shuts off. The 18g beeps at the end of a cycle, the OPI just turns off. The OPI has a HUGE countdown timer on top; the 18g has a small one in back where only the tech can see it. The OPI has four settings - 15, 30, 45 and 60 seconds; the 18g has three - 5, 20 and 30 seconds. Both do a great five finger cure, although the 18g seems to rely more on reflected light to get those awkward spaces on the sides of nails, especially thumbs, while the OPI has four LEDs set lower on the lamp and no reflective bottom. The 18g has a highly reflective interior top to bottom; the OPI is also reflective, but has no bottom.