Li-Ion-batteries have a lower self-discharge-rate as normal NiMH-batteries. There are low-self-discharge (LSD) NiMH-batteries like the Eneloop, that's what I prefer. The self-discharge-rates of Li-Ion-batteries and LSD-NiMH-batteries have about the same value. For my part I use both types in different situations. If you use protected Li-Ion-batteries and watch out how you handle them there is only a low chance of failure but I feel safer with have a NiMH-battery in my EDC-flashlight I carry on my body all day long. But if I want to step up the brightness and also keep the size rather compact I use a single-18650 flashlight. If you want to have the about the same advantages of 18650 Li-Ion-batteries in a smaller package you can always use 16340 sized Li-Ion-batteries.