I believe that they are the same thing - just like pastels are pastels, just different lines.
I doubt anyone could pick the butters from the lessers if they were all mixed together in the same tub. Many people describe butters as being brighter colored, but my lesser is one of the brightest that I've seen, and most butters I've seen at shows are consistently darker than lessers at the same show. Perhaps just a coincidence.
I probably will pick up a butter at some point and see for myself if there's much difference.