Li-Ion is a far superior to Nicad/Nimh in terms of self discharge. Nicad/Nimh lose 1 to 2% per day; after 3 months of just sitting, they are dead. Li-Ion lose 4 to 5% the first month of storage and 1 to 2% thereafter. Theorectical worse case for Li-Ion would 27% percent loss after one year, or to turn it around, 73% capacity remaining.
I've been playing around with Li-Ion quite a bit lately. And somtime around the end of the year I plan to toss a fully charged Li-Ion battery pack or two into the trunk of my car and let them sit for a full year. Want to do my own test.
Downside to Li-Ion: They can't deliver as much peak current as Nicad/Nimh. Li-Ion is O.K. for flashlight bulbs, up to a few amps of current. If your are running a heavy load such as an electric RC car or plane and need a 30 to 40amp discharge rate, Nicad is still your best choice.