Alkalines vs. good NiMHs will deliver a little higher energy content when used with low-current devices (about 4 W-hr vs. 3 W-hr). But when the alkalines start to go their voltage slowly drops over a long period of time. So the light will stay on for many more hours, but will slowly be getting dimmer and dimmer during that time. With NiMHs, the voltage stays pretty constant until the cells are almost dead and then drops much more rapidly.