It's not just the power, the NiCd loses ~25% of its charge per month just sitting around. That's a big deal for guys like me who might not touch a particular cordless tool for 6 months or more. I home something like 10 of them, but only my Porter Cable drill/driver // hammer/drill has Lithium batteries. Came with two batteries, I'm never left in the lurch.
I resisted Lithium rechargables for years because it's said they deteriorate faster at full charge than when at ~40% charge. I suppose that's true, but the upside makes them fabulous, at least for power tools. My DSLR uses 4 AAs and I use Eneloops for that and that works out great. My new Panasonic cordless telephone system uses NiMH's in the handsets, which works out great too but for my power cordless tools? Lithium >> NiCd or NiMH. End of Story.
My lithium packs don't self-discharge much at all, I've never tried to store them at 40% and they're always ready to use with plenty of juice left, though I do recharge them all at least once a year.
It can be a lot more than 25%/mo. loss for NiCd. I have an
old 14.4V Dewalt drill set, was their higher-end set at the time, rebuilt the battery packs about 8 years ago. Used Tenergy 2400mAh sub-C NiCd cells, vs the original (can't recall something like 1400mAh) Sanyos in the packs. They work fine, but even newly rebuilt they lost about 25% capacity within the first couple days, then less and less with further weeks, which is okay since it's not my primary use cordless drill.
Did it anyway because I had cleaned up the drill inside and out, checked brushes and bearings to be sure it was fit for another tour of duty, but also because it came in a kit with a nice snakelight that takes the same batteries, which I modded to convert to LED. Wish more manufacturers made this type of light, LED from the factory. It's quite large for the light output (though ~4X the output after LED conversion) but with a couple 3W LEDs it runs a day on a charge and I have smaller lights when needed. Main downside to the light is I didn't line the base with EMI shielding material so when it's on, the driver I put in the base interferes with FM radio reception.