Jim, my dad has new 18v (NiCd or NiMh, not sure which) dewalt, and they work almost as well as my 20v (LIon) but the LIon packs last almost forever, don't really loose charge from sitting, and the brushless versions are being rolled out. I got the brushless impact and I am waiting for dewalt to convert the circ saw to brushless. If I did not already have 2 dewalt 20v tools, I would go with Milwaukee as much more of their line, if not the entire line, is available in brushless. You get longer run time out of the brushless motors.
The longevity of the battery packs is where the Lithium stuff really shines, the older cordless tools always needed battery replacements which is why my Father-in-law would never buy one. Also if you didn't use one for a while, you had to charge it again BEFORE you started your project. After using my 20v drill, he got the drill/impact combo on black friday.