well, it depends on the application. Most of the time, applications do not require precisions in terms of power supply and currents. So I guess the difference b/w the LS and LM indexes are in terms of poewr supply and input resistance and propagation delay. Also, it difers maybe in the operating temperature and poewr dissipation. But I guess you don't need such data, or do you? (If you do, I could seek you a data sheet...)