They are both very nice lights, but the 550 is noticeably brighter (duh) , so we're keeping the 550s and returning the 400. Had we seen just the 400 we could have been perfectly happy with it. You know how it is once you start comparing things it's hard not to keep climbing up. But the brightness level and the coverage of the 550 was just undeniably better with a wider swath of the road covered and farther ahead as well.