Battery 101.... NiCd or Nickle Cadmium batteries are the cheapest form of rechargeble battery. But, they are a problem with equipment designed to use regular batteries as they are 1.2 V per cell as opposed to 1.5 for standard carbon cell battery. As well, they exhibit "memory". Simplified that means if they are not constantly totally discharged eventually they remember the lesser charges we give them. However, on the other hand, if you run them down flat they can be ruined too.
NiMH or nickle metal hydride batteries are/were designed to overcome the above problems. Higher voltage and supposed lack of memory effect. But,they need a different charger. Duracell makes a nice combination one.
If your unit is a high discharge device like a flash or motor drive, go to the NiMH. They are a better investment in the long run.
Now, lithium batteries (Lithium Ion to be correct) are the "cat's ass". But the cost will never justify them in normal use. They have a high discharge, good recovery rate and long time between recharge. And, they need a special charger.
Now, a really dirt cheap alternative. There are "enercell" green batteries, which are normally just alkalines that can be recharged. They are cheap, come with a charger pack to start and are direct replacements for alkalines. However, I find you will get about a year of use and recharge before they show lack of respect... I used them, because they were cheap. Dated them, and tossed them when they were a year old. But, eventually converted everything to NiMH. Personally I don't recommend them
It is all up to the amount of current that the device takes. Low current devices like light meters, radios, etc. can take just about anything. Extreme low current devices like watches used to take mercury, but environmental concerns changed that to silver halides. High current devices like flashes need the good stuff. Motor drives actually fall in the middle. And, for you digital folk, digital cameras are high current and need the good ones!
Tom
